CLN: Consistent imports in tests: pandas.util.testing as tm
This is a follow-up issue of #23018 with the goal of enforcing a consistent approach of importing things from pandas.util.testing inside the **test suite**.

Currently, there are multiple import styles used:
1. `import pandas.util.testing as tm`
2. `from pandas.util.testing import ...`
3. `from pandas.util import testing as tm`

As it is already the dominant import style used and also less prone to circular imports 1. is the prefered option. After migrating all current occurrences of 2. and 3. we'll add a linting rule that enforces this convention.

These are the current files that use 2. (generated with `grep -R -l --include="*.py" "from pandas.util.testing import " pandas/tests`:
- [x] pandas/tests/arrays/sparse/test_array.py #29307
- [x] pandas/tests/computation/test_eval.py #29307
- [x] pandas/tests/frame/test_api.py #29300
- [x] pandas/tests/frame/test_apply.py #29300
- [x] pandas/tests/frame/test_axis_select_reindex.py #29300
- [x] pandas/tests/frame/test_block_internals.py #29300
- [x] pandas/tests/frame/test_combine_concat.py #29300
- [x] pandas/tests/frame/test_dtypes.py #29300
- [x] pandas/tests/frame/test_indexing.py #29301
- [x] pandas/tests/frame/test_missing.py #29301
- [x] pandas/tests/frame/test_mutate_columns.py #29301
- [x] pandas/tests/frame/test_nonunique_indexes.py #29301
- [x] pandas/tests/frame/test_operators.py #29301
- [x] pandas/tests/frame/test_quantile.py #29301
- [x] pandas/tests/frame/test_query_eval.py #29302
- [x] pandas/tests/frame/test_rank.py #29302
- [x] pandas/tests/frame/test_replace.py #29302
- [x] pandas/tests/frame/test_reshape.py #29302
- [x] pandas/tests/frame/test_sort_values_level_as_str.py #29303
- [x] pandas/tests/frame/test_sorting.py #29303
- [x] pandas/tests/frame/test_timeseries.py #29303
- [x] pandas/tests/frame/test_to_csv.py #29303
- [x] pandas/tests/generic/test_frame.py #29307
- [x] pandas/tests/generic/test_generic.py #29307
- [x] pandas/tests/generic/test_series.py #29307
- [x] pandas/tests/groupby/test_categorical.py #29287
- [x] pandas/tests/groupby/test_counting.py #29287
- [x] pandas/tests/groupby/test_groupby.py #29287
- [x] pandas/tests/groupby/test_grouping.py  #29287
- [x] pandas/tests/groupby/test_index_as_string.py #29287
- [x] pandas/tests/groupby/test_nth.py #29287
- [x] pandas/tests/groupby/test_timegrouper.py #29287
- [x] pandas/tests/groupby/test_transform.py #29287
- [x] pandas/tests/indexes/datetimes/test_datetime.py #29289
- [x] pandas/tests/indexes/datetimes/test_tools.py #29289
- [x] pandas/tests/indexes/multi/test_astype.py #29289
- [x] pandas/tests/indexes/multi/test_indexing.py #29289
- [x] pandas/tests/indexes/test_base.py #29289
- [x] pandas/tests/indexes/test_category.py #29289
- [x] pandas/tests/indexes/timedeltas/test_partial_slicing.py #29289
- [x] pandas/tests/indexes/timedeltas/test_timedelta.py #29289
- [x] pandas/tests/indexes/timedeltas/test_tools.py #29289
- [x] pandas/tests/indexing/test_categorical.py #29291
- [x] pandas/tests/indexing/test_floats.py #29291
- [x] pandas/tests/indexing/test_iloc.py #29291
- [x] pandas/tests/indexing/test_indexing.py #29291
- [x] pandas/tests/indexing/test_ix.py #29291
- [x] pandas/tests/indexing/test_loc.py #29291
- [x] pandas/tests/internals/test_internals.py
- [x] pandas/tests/io/excel/test_openpyxl.py #29299
- [x] pandas/tests/io/excel/test_style.py #29299
- [x] pandas/tests/io/excel/test_writers.py #29299
- [x] pandas/tests/io/excel/test_xlrd.py #29299
- [x] pandas/tests/io/excel/test_xlsxwriter.py #29299
- [x] pandas/tests/io/excel/test_xlwt.py #29299
- [x] pandas/tests/io/json/test_compression.py #29299
- [x] pandas/tests/io/json/test_pandas.py #29299
- [x] pandas/tests/io/json/test_readlines.py #29299
- [x] pandas/tests/io/parser/test_textreader.py #29299
- [x] pandas/tests/io/pytables/test_compat.py #29299
- [x] pandas/tests/io/pytables/test_complex.py #29299
- [x] pandas/tests/io/pytables/test_store.py #29299
- [x] pandas/tests/io/pytables/test_timezones.py #29299
- [x] pandas/tests/io/test_clipboard.py #29299
- [x] pandas/tests/io/test_feather.py #29299
- [x] pandas/tests/io/test_gcs.py #29299
- [x] pandas/tests/io/test_html.py #29299
- [x] pandas/tests/io/test_packers.py #29299
- [x] pandas/tests/plotting/common.py #29307
- [x] pandas/tests/plotting/test_datetimelike.py #29307
- [x] pandas/tests/resample/test_base.py #29286
- [x] pandas/tests/resample/test_datetime_index.py #29286
- [x] pandas/tests/resample/test_period_index.py #29286
- [x] pandas/tests/resample/test_resample_api.py #29286
- [x] pandas/tests/resample/test_resampler_grouper.py #29286
- [x] pandas/tests/resample/test_time_grouper.py #29286
- [x] pandas/tests/resample/test_timedelta.py #29286
- [x] pandas/tests/reshape/merge/test_join.py #29285
- [x] pandas/tests/reshape/merge/test_merge.py #29285
- [x] pandas/tests/reshape/merge/test_merge_asof.py #29285
- [x] pandas/tests/reshape/merge/test_merge_index_as_string.py #29285
- [x] pandas/tests/reshape/merge/test_merge_ordered.py #29285
- [x] pandas/tests/reshape/test_concat.py #29285
- [x] pandas/tests/reshape/test_reshape.py #29285
- [x] pandas/tests/series/indexing/test_alter_index.py #29277
- [x] pandas/tests/series/indexing/test_boolean.py #29277
- [x] pandas/tests/series/indexing/test_datetime.py #29277
- [x] pandas/tests/series/indexing/test_iloc.py #29277
- [x] pandas/tests/series/indexing/test_indexing.py #29277
- [x] pandas/tests/series/indexing/test_loc.py #29277
- [x] pandas/tests/series/indexing/test_numeric.py #29277
- [x] pandas/tests/series/test_analytics.py #29273
- [x] pandas/tests/series/test_api.py #29273
- [x] pandas/tests/series/test_apply.py #29273
- [x] pandas/tests/series/test_combine_concat.py #29273
- [x] pandas/tests/series/test_constructors.py #29273
- [x] pandas/tests/series/test_datetime_values.py #29273
- [x] pandas/tests/series/test_internals.py #29273
- [x] pandas/tests/series/test_io.py #29273
- [x] pandas/tests/series/test_missing.py #29273
- [x] pandas/tests/series/test_operators.py #29273
- [x] pandas/tests/series/test_rank.py #29273
- [x] pandas/tests/series/test_sorting.py #29273
- [x] pandas/tests/series/test_timeseries.py #29273
- [x] pandas/tests/test_algos.py #29307
- [x] pandas/tests/test_expressions.py #29307
- [x] pandas/tests/test_join.py #29307
- [x] pandas/tests/test_sorting.py #29307
- [x] pandas/tests/test_strings.py #29307
- [x] pandas/tests/util/test_assert_almost_equal.py #29278
- [x] pandas/tests/util/test_assert_categorical_equal.py #29278
- [x] pandas/tests/util/test_assert_extension_array_equal.py #29278
- [x] pandas/tests/util/test_assert_frame_equal.py #29278
- [x] pandas/tests/util/test_assert_index_equal.py #29278
- [x] pandas/tests/util/test_assert_interval_array_equal.py #29278
- [x] pandas/tests/util/test_assert_numpy_array_equal.py #29278
- [x] pandas/tests/util/test_assert_series_equal.py #29278
