text: Title: When following raw URL recipe based on RAW_URI, it breaks routing in TestClient
  Body: I'm building REST API with many %-encoded slashes in URI (i.e., as data, not path separator). For example, request URI `/book/baking-book/document/sweet%2Fcheescake.md` will never reach the route declared as `/book/{book}/document/{path}`.
  
  I understand that's not Falcon's fault, but the WSGI server, which "eats" %-encoding before providing it to Falcon in the `PATH_INFO` CGI variable. Thank you for the well-explained reason in FAQ and [decoding raw URL recipe](https://falcon.readthedocs.io/en/stable/user/recipes/raw-url-path.html) based on Gunicorn's `RAW_URI` non-standard variable.
  
  However, if I follow this recipe, it breaks all tests because [TestClient hard-code `RAW_URI` to `/`](https://github.com/falconry/falcon/blob/master/falcon/testing/helpers.py#L1197), so no URI will route properly anymore.