Merge document-test-class-names-7032: Document the recommended naming of
test classes in Twisted.

Author: eeshangarg
Reviewers: khorn
Fixes: #7032

Add some documentation to the Testing Standard which specifies the correct
way of naming test classes.

git-svn-id: svn://svn.twistedmatrix.com/svn/Twisted/trunk@43075 bbbe8e31-12d6-0310-92fd-ac37d47ddeeb