Provide an easy way to test sending of signals

[James Bennett]

Currently, when writing tests for an application that sends signals, there's no easy built-in way to test the sending of signals.

A simple solution would be to add a context-manager assertion that can check a signal was sent and received keyword arguments with certain names, and then further inspection of the received arguments could be done inside the with block. For example:

with self.AssertSignalSent(some_signal, required_kwargs=['my_kwarg']):
    # ...do stuff...

[Tim Graham]

WIP ​PR

[Simon Charette]

The code is looking good. Let's wait for a documentation review from a native speaker.

[James Bennett]

We're now at four other members of -core reviewed, all concerns addressed. Speak now or implement fixes for your critique later.

[Tim Graham]

I left some comments on the PR. When James is finished with the updates, I'll add a commit to the branch that makes use of this assertion in Django's test suite.