we randomly choose between sms providers now - this means that tests may
sometimes send firetext and sometimes mmg, so we'd need to patch out
different HTTP calls, expect different values in sent_by, etc etc.
To ensure tests are consistent, add a new fixture that is always used by
notify_db_session, which sets the priorities of the sms providers to
100% mmg 0% firetext. if you need to test other values, then you should
set the values manually in the test file
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