pythonGH-113528: pathlib ABC tests: add repr to dummy path classes.

The `DummyPurePath` and `DummyPath` test classes are simple subclasses of
`PurePathBase` and `Pathbase`. This commit adds `__repr__()` methods to the
dummy classes, which makes debugging test failures less painful.

Lib/test/test_pathlib/test_pathlib_abc.py

@@ -51,6 +51,9 @@ def __eq__(self, other):
     def __hash__(self):
         return hash(str(self))
 
+    def __repr__(self):
+        return "{}({!r})".format(self.__class__.__name__, self.as_posix())
+
 
 class DummyPurePathTest(unittest.TestCase):
     cls = DummyPurePath
@@ -719,6 +722,9 @@ def __eq__(self, other):
     def __hash__(self):
         return hash(str(self))
 
+    def __repr__(self):
+        return "{}({!r})".format(self.__class__.__name__, self.as_posix())
+
     def stat(self, *, follow_symlinks=True):
         if follow_symlinks:
             path = str(self.resolve())