CI/TST: slow tests
xref #30641

from the new arm64 job, but indicative of general slowness: https://travis-ci.org/github/pandas-dev/pandas/jobs/685955999

- let's make these much faster
- we can still mark some as `slow` if needed
- let's change the https://github.com/pandas-dev/pandas/blob/master/ci/run_tests.sh#L23, `durations` to 20 or 30

This issue can be closed by multiple PRs

```
========================== slowest 10 test durations ===========================
88.88s call     pandas/tests/resample/test_datetime_index.py::test_nearest_upsample_with_limit[tzlocal()-30S-Y]
65.15s call     pandas/tests/resample/test_datetime_index.py::test_nearest_upsample_with_limit[tzlocal()-30S-10M]
22.33s call     pandas/tests/io/parser/test_multi_thread.py::test_multi_thread_path_multipart_read_csv[python]
21.54s call     pandas/tests/io/parser/test_multi_thread.py::test_multi_thread_path_multipart_read_csv[c_high]
21.03s call     pandas/tests/io/parser/test_multi_thread.py::test_multi_thread_path_multipart_read_csv[c_low]
19.46s call     pandas/tests/resample/test_datetime_index.py::test_nearest_upsample_with_limit['dateutil/US/Pacific'-30S-Y]
15.75s call     pandas/tests/resample/test_datetime_index.py::test_nearest_upsample_with_limit['dateutil/Asia/Singapore'-30S-10M]
15.71s call     pandas/tests/resample/test_datetime_index.py::test_nearest_upsample_with_limit['dateutil/Asia/Singapore'-30S-Y]
15.61s call     pandas/tests/resample/test_datetime_index.py::test_nearest_upsample_with_limit['Asia/Tokyo'-30S-Y]
15.59s call     pandas/tests/resample/test_datetime_index.py::test_nearest_upsample_with_limit['US/Eastern'-30S-Y]
14.27s call     pandas/tests/resample/test_datetime_index.py::test_nearest_upsample_with_limit['Asia/Tokyo'-30S-10M]
13.18s call     pandas/tests/groupby/transform/test_transform.py::test_cython_transform_frame[shift-args3-<lambda>]
13.11s call     pandas/tests/test_sorting.py::TestSorting::test_int64_overflow_moar
12.98s call     pandas/tests/resample/test_datetime_index.py::test_nearest_upsample_with_limit['US/Eastern'-30S-10M]
12.92s call     pandas/tests/resample/test_datetime_index.py::test_nearest_upsample_with_limit['dateutil/US/Pacific'-30S-10M]
12.78s call     pandas/tests/io/sas/test_sas7bdat.py::TestSAS7BDAT::test_iterator_loop
12.76s call     pandas/tests/io/sas/test_xport.py::TestXport::test1_basic
12.18s call     pandas/tests/groupby/transform/test_transform.py::test_cython_transform_frame[shift-args2-<lambda>]
11.97s call     pandas/tests/indexing/multiindex/test_chaining_and_caching.py::test_indexer_caching
11.95s call     pandas/tests/resample/test_datetime_index.py::test_nearest_upsample_with_limit[pytz.FixedOffset(300)-30S-Y]
11.28s call     pandas/tests/io/parser/test_multi_thread.py::test_multi_thread_string_io_read_csv[python]
11.19s call     pandas/tests/resample/test_datetime_index.py::test_nearest_upsample_with_limit[datetime.timezone(datetime.timedelta(0, 3600))-30S-Y]
10.60s call     pandas/tests/resample/test_datetime_index.py::test_nearest_upsample_with_limit[tzutc()-30S-Y]
10.45s call     pandas/tests/resample/test_datetime_index.py::test_nearest_upsample_with_limit[<UTC>-30S-Y]
10.35s call     pandas/tests/resample/test_datetime_index.py::test_nearest_upsample_with_limit[datetime.timezone.utc-30S-Y]
10.19s call     pandas/tests/resample/test_datetime_index.py::test_nearest_upsample_with_limit[datetime.timezone(datetime.timedelta(-1, 82800), 'foo')-30S-Y]
10.12s call     pandas/tests/resample/test_datetime_index.py::test_nearest_upsample_with_limit[tzutc()-30S-10M]
10.05s call     pandas/tests/resample/test_datetime_index.py::test_nearest_upsample_with_limit['UTC'-30S-Y]
9.99s call     pandas/tests/resample/test_datetime_index.py::test_nearest_upsample_with_limit[pytz.FixedOffset(-300)-30S-Y]
9.58s call     pandas/tests/resample/test_datetime_index.py::test_nearest_upsample_with_limit[<UTC>-30S-10M]```