text: Title: Add `__repr__` to test Result
  Body: At the time of this writing, [`falcon.testing.Result`](https://falcon.readthedocs.io/en/stable/api/testing.html#falcon.testing.Result) has no custom `__repr__`:
  ```python
  >>> import falcon
  >>> import falcon.testing
  >>> app = falcon.App()
  >>> client = falcon.testing.TestClient(app)
  >>> result = client.get('/something')
  >>> result
  <falcon.testing.client.Result object at 0x7f292ce046d0>
  ```
  
  `__repr__` is also picked up and shown by the popular `pytest`.
  
  It could be nice to add a representation with a concise summary describing the most important attributes of the result (status code etc).