Make the test pass by making SecondaryAuthority a FileAuthority subclass.

Subclassing is terrible but it's better than what this code did before!


git-svn-id: svn://svn.twistedmatrix.com/svn/Twisted/branches/secondary-dns-lookup-error-7408@43047 bbbe8e31-12d6-0310-92fd-ac37d47ddeeb