REF: use single-test-class for datetimetz, period tests
- [ ] closes #xxxx (Replace xxxx with the GitHub issue number)
- [ ] [Tests added and passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#writing-tests) if fixing a bug or adding a new feature
- [ ] All [code checks passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit).
- [ ] Added [type annotations](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#type-hints) to new arguments/methods/functions.
- [ ] Added an entry in the latest `doc/source/whatsnew/vX.X.X.rst` file if fixing a bug or adding a new feature.

```bash
mypy.....................................................................Failed
- hook id: mypy
- duration: 127.2s
- exit code: 1

pandas/tests/extension/test_period.py:95: error: Item "dtype[Any]" of "dtype[Any] | ExtensionDtype" has no attribute "freq"  [union-attr]
pandas/tests/extension/test_period.py:95: error: Item "ExtensionDtype" of "dtype[Any] | ExtensionDtype" has no attribute "freq"  [union-attr]
pandas/tests/extension/test_datetime.py:131: error: Item "dtype[Any]" of "dtype[Any] | ExtensionDtype" has no attribute "tz"  [union-attr]
pandas/tests/extension/test_datetime.py:131: error: Item "ExtensionDtype" of "dtype[Any] | ExtensionDtype" has no attribute "tz"  [union-attr]
Found 4 errors in 2 files (checked 1413 source files)
``` updated+green Thanks @jbrockmendel 