test: send messages to banned accounts

Better test coverage of banned accounts related features.
We try to follow the scenario from the wiki:
https://tuleap.ring.cx/plugins/mediawiki/wiki/ring/index.php?title=Banned_contact



Also, fix various issues in the mocking/test system, general
tests cleanup.

Change-Id: I6ca8257425b75c2b9f7c9c4db654a55f4953935a
Reviewed-by: Sebastien Blin <sebastien.blin@savoirfairelinux.com>