pythonGH-130614: pathlib ABCs: revise test suite for path joining (python#130988)

Test `pathlib.types._JoinablePath` in a dedicated test module. These tests
cover `LexicalPath`, `PurePath` and `Path`, where `LexicalPath` is defined
in a new `test.test_pathlib.support` package.

Author: barneygale

Lib/test/test_pathlib/support/__init__.py

@@ -0,0 +1,33 @@
+"""
+Simple implementation of JoinablePath, for use in pathlib tests.
+"""
+
+import os.path
+import pathlib.types
+
+
+class LexicalPath(pathlib.types._JoinablePath):
+    __slots__ = ('_segments',)
+    parser = os.path
+
+    def __init__(self, *pathsegments):
+        self._segments = pathsegments
+
+    def __hash__(self):
+        return hash(str(self))
+
+    def __eq__(self, other):
+        if not isinstance(other, LexicalPath):
+            return NotImplemented
+        return str(self) == str(other)
+
+    def __str__(self):
+        if not self._segments:
+            return ''
+        return self.parser.join(*self._segments)
+
+    def __repr__(self):
+        return f'{type(self).__name__}({str(self)!r})'
+
+    def with_segments(self, *pathsegments):
+        return type(self)(*pathsegments)