GH-112675: Move path joining tests into `test_posixpath` and `test_ntpath` (#112676)

In `test_pathlib`, the `check_drive_root_parts` test methods evaluated
both joining and parsing/normalisation of paths. This dates from a time
when pathlib implemented both functions itself, but nowadays path joining
is done with `posixpath.join()` and `ntpath.join()`.

This commit moves the joining-related test cases into `test_posixpath` and
`test_ntpath`.