GH-130614: pathlib ABCs: revise test suite for Windows path joining (#131016)

Test Windows-flavoured `pathlib.types._JoinablePath` in a dedicated test
module. These tests cover `LexicalWindowsPath`, `PureWindowsPath` and
`WindowsPath`, where `LexicalWindowsPath` is a simple implementation of
`_JoinablePath` for use in tests.