Split large test modules
- [ ] pandas/tests/tseries/offsets/test_offsets.py #27085

- [ ] pandas/tests/test_resample.py #17806 (has now been split, some post split cleanup required)

- [ ] pandas/tests/io/test_pytables.py #18498

- [x] pandas/tests/test_window.py #19228

- [ ] pandas/tests/test_base.py #23877

- [x] pandas/tests/frame/test_indexing.py #25798

PRs welcome!