text: Title: falcon.testing.* is not (re-)exported
  Body: Now that falcon 4 with type hints has been released 🎉 , we've enabled type checking using mypy as well for the falcon namespace for our codebase.
  
  Unfortunately, `falcon.testing.__init__` does not reexport the names it imports, which causes complaints by mypy, for example:
  ```
  tests/test_subscription_data.py:13: error: Name "falcon.testing.TestClient" is not defined  [name-defined]
  ```
  
  (PR will follow)
  