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.

Lib/test/test_pathlib/support/lexical_path.py

@@ -4,6 +4,7 @@
 
 import os.path
 import pathlib.types
+import posixpath
 
 
 class LexicalPath(pathlib.types._JoinablePath):
@@ -31,3 +32,8 @@ def __repr__(self):
 
     def with_segments(self, *pathsegments):
         return type(self)(*pathsegments)
+
+
+class LexicalPosixPath(LexicalPath):
+    __slots__ = ()
+    parser = posixpath