CI: mark window online test slow
Seems like the new online tests added increase test runtime a lot, making failures like https://dev.azure.com/pandas-dev/pandas/_build/results?buildId=61437&view=logs&j=404760ec-14d3-5d48-e580-13034792878f&t=f81e4cc8-d61a-5fb8-36be-36768e5c561a and 
https://dev.azure.com/pandas-dev/pandas/_build/results?buildId=61437&view=logs&j=404760ec-14d3-5d48-e580-13034792878f&t=f81e4cc8-d61a-5fb8-36be-36768e5c561a
more common. Slowest durations showing this are below, have marked these as slow as a potential solution
```
============================ slowest 30 durations =============================
99.91s call     pandas/tests/resample/test_base.py::test_resample_interpolate[period_range-pi-_index_start1-_index_end1]
47.56s call     pandas/tests/resample/test_period_index.py::TestPeriodIndex::test_weekly_upsample[end-D-FRI]
47.36s call     pandas/tests/window/test_online.py::TestEWM::test_online_vs_non_online_mean[True-True-True-True-True-obj1]
45.55s teardown pandas/tests/window/test_rolling.py::test_rolling_zero_window
30.25s call     pandas/tests/tslibs/test_timezones.py::test_cache_keys_are_distinct_for_pytz_vs_dateutil[America/Argentina/San_Luis]
28.37s call     pandas/tests/window/test_online.py::TestEWM::test_online_vs_non_online_mean[True-True-True-True-True-obj0]
23.03s call     pandas/tests/window/test_online.py::TestEWM::test_online_vs_non_online_mean[True-True-False-True-True-obj1]
23.00s call     pandas/tests/window/test_online.py::TestEWM::test_online_vs_non_online_mean[True-True-True-False-True-obj1]
20.95s call     pandas/tests/window/test_online.py::TestEWM::test_online_vs_non_online_mean[True-True-True-False-True-obj0]
20.30s call     pandas/tests/window/test_online.py::TestEWM::test_online_vs_non_online_mean[True-True-False-True-True-obj0]
19.94s call     pandas/tests/window/test_online.py::TestEWM::test_online_vs_non_online_mean[True-True-True-True-False-obj1]
19.81s call     pandas/tests/window/test_online.py::TestEWM::test_online_vs_non_online_mean[True-True-False-True-False-obj0]
19.37s call     pandas/tests/window/test_online.py::TestEWM::test_online_vs_non_online_mean[True-True-True-False-False-obj1]
19.25s call     pandas/tests/window/test_online.py::TestEWM::test_online_vs_non_online_mean[True-True-True-False-False-obj0]
18.13s call     pandas/tests/test_sorting.py::TestSorting::test_int64_overflow_moar
18.10s call     pandas/tests/window/test_online.py::TestEWM::test_online_vs_non_online_mean[True-True-True-True-False-obj0]
18.07s call     pandas/tests/window/test_online.py::TestEWM::test_online_vs_non_online_mean[True-True-False-True-False-obj1]
17.17s call     pandas/tests/window/test_numba.py::TestEngine::test_numba_vs_cython_apply[True-False-False-False-True]
17.16s call     pandas/tests/window/test_online.py::TestEWM::test_online_vs_non_online_mean[True-True-False-False-True-obj1]
16.84s call     pandas/tests/window/test_online.py::TestEWM::test_online_vs_non_online_mean[False-True-True-False-True-obj1]
16.75s call     pandas/tests/window/test_online.py::TestEWM::test_online_vs_non_online_mean[True-True-False-False-True-obj0]
16.48s call     pandas/tests/window/test_online.py::TestEWM::test_online_vs_non_online_mean[False-True-True-True-True-obj1]
16.47s setup    pandas/tests/io/test_fsspec.py::test_from_s3_csv
16.36s call     pandas/tests/window/test_online.py::TestEWM::test_online_vs_non_online_mean[False-True-False-True-True-obj1]
15.93s call     pandas/tests/io/test_compression.py::test_with_missing_lzma
15.83s call     pandas/tests/window/test_online.py::TestEWM::test_online_vs_non_online_mean[False-True-False-False-True-obj1]
15.13s call     pandas/tests/window/test_online.py::TestEWM::test_online_vs_non_online_mean[True-True-False-False-False-obj1]
14.46s call     pandas/tests/window/test_online.py::TestEWM::test_online_vs_non_online_mean[False-True-True-True-True-obj0]
14.14s call     pandas/tests/window/test_online.py::TestEWM::test_online_vs_non_online_mean[True-True-False-False-False-obj0]
14.00s call     pandas/tests/window/test_online.py::TestEWM::test_online_vs_non_online_mean[False-True-False-True-False-obj0]
```
