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/test_join_posix.py

@@ -0,0 +1,49 @@
+"""
+Tests for Posix-flavoured pathlib.types._JoinablePath
+"""
+
+import os
+import unittest
+
+from pathlib import PurePosixPath, PosixPath
+from test.test_pathlib.support.lexical_path import LexicalPosixPath
+
+
+class JoinTestBase:
+    def test_join(self):
+        P = self.cls
+        p = P('//a')
+        pp = p.joinpath('b')
+        self.assertEqual(pp, P('//a/b'))
+        pp = P('/a').joinpath('//c')
+        self.assertEqual(pp, P('//c'))
+        pp = P('//a').joinpath('/c')
+        self.assertEqual(pp, P('/c'))
+
+    def test_div(self):
+        # Basically the same as joinpath().
+        P = self.cls
+        p = P('//a')
+        pp = p / 'b'
+        self.assertEqual(pp, P('//a/b'))
+        pp = P('/a') / '//c'
+        self.assertEqual(pp, P('//c'))
+        pp = P('//a') / '/c'
+        self.assertEqual(pp, P('/c'))
+
+
+class LexicalPosixPathJoinTest(JoinTestBase, unittest.TestCase):
+    cls = LexicalPosixPath
+
+
+class PurePosixPathJoinTest(JoinTestBase, unittest.TestCase):
+    cls = PurePosixPath
+
+
+if os.name != 'nt':
+    class PosixPathJoinTest(JoinTestBase, unittest.TestCase):
+        cls = PosixPath
+
+
+if __name__ == "__main__":
+    unittest.main()