GH-130614: pathlib ABCs: revise test suite for Posix path joining (#131017)

Test Posix-flavoured `pathlib.types._JoinablePath` in a dedicated test
module. These tests cover `LexicalPosixPath`, `PurePosixPath` and
`PosixPath`, where `LexicalPosixPath` is a simple implementation of
`_JoinablePath` for use in tests.