Fixed #16990 -- Fixed a couple of small docstring typos in the...

Fixed #16990 -- Fixed a couple of small docstring typos in the `django/test/testcases.py` module and did some minor cleanup while I was in the area.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@16928 bcc190cf-cafb-0310-a4f2-bffc1f526a37