Split up the test with multiple assertions into several tests.

Also adjust the bytes/unicode usage in these tests.
The name of a type is a native string.
The string version of an exception is a native string.

Also be explicit about the expected strings instead of re-computing them the same way
as the implementation computes them.  This means the test will probably fail on Python 3
and will need to be adjusted when Banana is for-real ported to Python 3.

refs #7663


git-svn-id: svn://svn.twistedmatrix.com/svn/Twisted/branches/banana-unsupported-type-exception-7663@43207 bbbe8e31-12d6-0310-92fd-ac37d47ddeeb