GH-89812: Clean up pathlib tests.

[barneygale]

Merge PurePathTest into _BasePurePathTest, and PathTest into _BasePathTest. No change of coverage.

• Issue: Add tarfile.TarPath #89812

[barneygale]

Note that the _BasePurePathTest and _BasePathTest classes reflect a time long ago when pathlib had pathlib._BasePurePath and pathlib._BasePath classes. These haven't existed since pathlib was an independent package. We may re-introduce them in future (see #104810), but they will look rather different!

[ambv]

This depends on GH-105744.

[ambv]

I currently don't have a Windows box to test the coverage with, but I tested on macOS and Linux and this looks good to me. Let's go for it! Thanks for the cleanup, Barney! ✨ 🍰 ✨

[ambv]

(If you're curious to check on Windows, the guide is good: https://devguide.python.org/testing/coverage/)

[barneygale]

Thank you for the review, Łukasz!