[test] Gather tests about mail module From sobjects/test/unittest_notification.py to test/unittest_mail.py where other tests for mail.py are already. Some style fixes along the way.

etc/init.d
etc/cubicweb.d
etc/bash_completion.d
usr/bin
usr/share/doc/cubicweb-ctl
var/log/cubicweb
var/lib/cubicweb/backup
var/lib/cubicweb/instances