REF: use single-test-class for datetimetz, period tests

[jbrockmendel]

• closes #xxxx (Replace xxxx with the GitHub issue number)
• Tests added and passed if fixing a bug or adding a new feature
• All code checks passed.
• Added type annotations 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.

[mroeschke]

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)

[jbrockmendel]

updated+green

[mroeschke]

Thanks @jbrockmendel