Add an assertMessages assertion

[François Freitag]

Most projects I’ve worked on used the messages framework, and many of them were repeating some boilerplate in the form described in ​https://stackoverflow.com/questions/2897609/how-can-i-unit-test-django-messages.

from django.contrib.messages import get_messages

messages = list(get_messages(response.wsgi_request))
self.assertEqual(len(messages), 1)
self.assertEqual(str(messages[0]), 'my message')

I see several small pain points in this code:

• accessing response.wsgi_request, because get_messages needs a request but all we have is a response.
• get_messages returns an iterator, that must be consumed to perform assertions, hence the casting to a list
• the expectation lacks precision (missing the level) and clarity
• it’s boilerplate-ish

Maybe adding an assertion to facilitate this test would benefit the other projects.

The code is small enough that it won’t be worth pulling down as a 3rd party package, especially since it requires changing the base test class over the entire test suite.