Fix asyncnotifier unit tests

On my workstation, if I run the following two tests in this exact
order:

- TestSingleFileEventHandler.testError
- TestSingleFileEventHandler.testReplace

the second test will fail, because there is no "unregister" of
previous tests (and the poll() done by asyncore will fire on the
previous files which are now removed, etc. etc.).

While investigating this, I realised that we don't do any cleanup of
the various handlers/notifiers we setup for this test, so if we simply
add a proper cleanup, the tests will work nicely (in any order :).

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: René Nussbaumer <rn@google.com>