tests: fix test_cwd_snapshot

Without restoring the cwd, successive tests might fail to parse the
config (via `_pytest.config._prepareconfig()`, for when `--lsof` is
used).

And it is good practice to restore the cwd in any case anyway.

src/_pytest/pytester.py

@@ -527,6 +527,8 @@ class Testdir:
 
     """
 
+    __test__ = False
+
     CLOSE_STDIN = object
 
     class TimeoutExpired(Exception):