Add test capturing expectation revealed by #253.

Author: jaraco

importlib_resources/tests/test_reader.py

@@ -76,6 +76,10 @@ def test_join_path(self):
             os.path.join('namespacedata01', 'imaginary'),
         )
 
+    def test_join_path_compound(self):
+        path = MultiplexedPath(self.folder)
+        assert not path.joinpath('imaginary/foo.py').exists()
+
     def test_repr(self):
         self.assertEqual(
             repr(MultiplexedPath(self.folder)),