Fixed #19885 - cleaned up the django.test namespace

Common testing tools - in particular, the ones referenced in the tutorial
and in the testing overview - were made importable from `django.test`.
The documentation was cleaned up to consistently refer to these
tools as being part of `django.test`.

Note that I did *not* adjust all the internal references (in Django's own test code) to use this path. I can do that, but I'm not sure if it's necessary or desirable.

See [ticket #19885](https://code.djangoproject.com/ticket/19885).