CI: Mark slow tests as slow to speed up ci
We have a few tests which are really slow and not marked as such. I went through the list below and marked everything greater than 4 seconds as slow. Should give us between 4 and 5 minutes performance boost

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# 29.68s call     pandas/tests/io/sas/test_sas7bdat.py::TestSAS7BDAT::test_iterator_loop
# 12.98s call     pandas/tests/test_sorting.py::TestSorting::test_int64_overflow_moar
# 11.43s call     pandas/tests/indexing/multiindex/test_chaining_and_caching.py::test_indexer_caching
# 10.13s call     pandas/tests/groupby/transform/test_transform.py::test_cython_transform_frame[shift-args3-<lambda>]
# 9.87s call     pandas/tests/io/sas/test_sas7bdat.py::TestSAS7BDAT::test_from_iterator
# 9.79s call     pandas/tests/groupby/transform/test_transform.py::test_cython_transform_frame[shift-args2-<lambda>]
# 9.41s teardown pandas/tests/window/moments/test_moments_rolling_quantile.py::test_center_reindex_frame[1.0]
# 8.82s call     pandas/tests/io/sas/test_xport.py::TestXport::test1_basic
# 8.43s call     pandas/tests/io/parser/common/test_chunksize.py::test_warn_if_chunks_have_mismatched_type[python]
# 8.38s call     pandas/tests/io/parser/common/test_chunksize.py::test_chunks_have_consistent_numerical_type[python]
# 8.13s call     pandas/tests/io/parser/test_c_parser_only.py::test_grow_boundary_at_cap[c_low]
# 8.12s call     pandas/tests/io/parser/test_c_parser_only.py::test_grow_boundary_at_cap[c_high]
# 6.97s call     pandas/tests/indexes/datetimes/test_date_range.py::TestDateRanges::test_date_range_int64_overflow_stride_endpoint_different_signs
# 5.90s call     pandas/tests/frame/test_constructors.py::TestDataFrameConstructors::test_constructor_mrecarray
# 5.84s teardown pandas/tests/window/moments/test_moments_rolling_skew_kurt.py::test_center_reindex_frame[skew]
# 5.61s call     pandas/tests/io/parser/test_c_parser_only.py::test_precise_conversion[c_high]
# 5.29s call     pandas/tests/io/sas/test_sas7bdat.py::TestSAS7BDAT::test_path_pathlib
# 5.13s call     pandas/tests/groupby/transform/test_transform.py::test_cython_transform_frame[cumsum-args1-<lambda>]
# 4.93s call     pandas/tests/io/sas/test_sas7bdat.py::TestSAS7BDAT::test_from_file
# 4.86s call     pandas/tests/io/sas/test_sas7bdat.py::TestSAS7BDAT::test_from_buffer
# 4.84s call     pandas/tests/io/sas/test_sas7bdat.py::TestSAS7BDAT::test_path_localpath
# 4.59s call     pandas/tests/indexes/multi/test_integrity.py::test_hash_collisions
# 4.26s call     pandas/tests/indexing/interval/test_interval.py::TestIntervalIndex::test_loc_getitem_large_series
# 3.99s call     pandas/tests/io/parser/test_c_parser_only.py::test_precise_conversion[c_low]

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