The patch adds too much.  Some of the comments are too obvious.  See PEP 8.  Many of the messages repeat the information already provided in the custom messages already provided by the assertXyz failure methods.  For instance, when assertEqual(a, b) fails, the message already prints out something like "%s != $s" % (repr(a), repr(b)).  Perhaps you were not familiar with this.  One of the problems of retrofitting tests is that we do not see the default failure messages.

In any case, over-commenting and over-messaging are not harmless as they make code and failure messages harder, not easier to read.  I will review and add what I think is useful.