text: Title: Deprecate `falcon.testing.httpnow()`
  Body: Apparently this ancient alias has survived unnoticed from the very early iterations of the framework.
  
  Wrap it in the [`@deprecated(...)`](https://falcon.readthedocs.io/en/stable/api/util.html#falcon.util.deprecated) decorator so that we don't miss it again.
  
  Write that it will be removed in Falcon 5.0 in the deprecation message.