Merge fix-si-warning-7097

Author: hynek, exarkun
Reviewer: exarkun, glyph
Fixes: #7097

Add test coverage for the warning emitted when dependencies necessary for
correct "service identity" checking in the TLS implementation are missing.
Also fix a few minor issues with those warnings.


git-svn-id: svn://svn.twistedmatrix.com/svn/Twisted/trunk@42594 bbbe8e31-12d6-0310-92fd-ac37d47ddeeb