Merge mail-tests-names-7708

Author: eeshangarg
Reviewer: exarkun
Fixes: #7708

Rename all of the test case classes in Twisted Mail to conform to the
recently documented test case class naming convention.


git-svn-id: svn://svn.twistedmatrix.com/svn/Twisted/trunk@43518 bbbe8e31-12d6-0310-92fd-ac37d47ddeeb