docstring_parser 0.18 excludes its tests from built wheels, so
`docstring_parser.__path__[0] / 'tests'` does not exist and test
collection fails with FileNotFoundError. Skip generating the cross-check
test cases when the directory is unavailable.

--- a/tests/test_linter/test_extractors/test_exceptions.py
+++ b/tests/test_linter/test_extractors/test_exceptions.py
@@ -293,6 +293,8 @@ def get_docstring_parser_tests():
     tests_found = 0
     if astroid is None:
         return
+    if not root.is_dir():
+        return
     for path in root.iterdir():
         if path.suffix != '.py':
             continue