CLN: move pytables tests to tests/io/pytables dir
This is a follow-up to [this comment](https://github.com/pandas-dev/pandas/pull/26818#issuecomment-504443559).

This moves two test modules (`test_pytables.py` and `test_pytables_missing.py`) into the new `tests/io/pytables` directory introduced in https://github.com/pandas-dev/pandas/pull/26818.

@jreback let me know if I should create an issue for this first.