By "writing a test" I mean adding a unittest-based test case in Lib/test/test_asynchat.py which fails before fixing Lib/asynchat.py and succeeds afterwards.

Now, I'm not even sure I properly understood your bug exactly.
I've never used negative terminators in asynchat and I'm not even sure why one would want to. 

In this case, taking a look at a test would help me (and others) to understand what you are complaining about exactly and figure out whether this is actually a bug and if it is worth fixing it.

As for how to properly write a patch see:
http://www.python.org/dev/faq/
All new patches should be applied to python 3.3 first so every time you submit a new one you should work on the 3.3 branch (current trunk) which is:
http://svn.python.org/projects/python/branches/py3k/