make tests use mmg 100% of the time by default

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

tests/app/dao/test_provider_details_dao.py

@@ -224,17 +224,17 @@ def test_dao_get_provider_stats(notify_db_session):
     assert result[0].created_by_name is None
     assert result[0].current_month_billable_sms == 0
 
-    assert result[1].identifier == 'mmg'
-    assert result[1].display_name == 'MMG'
-    assert result[1].supports_international is True
+    assert result[1].identifier == 'firetext'
+    assert result[1].notification_type == 'sms'
+    assert result[1].supports_international is False
     assert result[1].active is True
-    assert result[1].current_month_billable_sms == 4
+    assert result[1].current_month_billable_sms == 5
 
-    assert result[2].identifier == 'firetext'
-    assert result[2].notification_type == 'sms'
-    assert result[2].supports_international is False
+    assert result[2].identifier == 'mmg'
+    assert result[2].display_name == 'MMG'
+    assert result[2].supports_international is True
     assert result[2].active is True
-    assert result[2].current_month_billable_sms == 5
+    assert result[2].current_month_billable_sms == 4
 
     assert result[3].identifier == 'dvla'
     assert result[3].current_month_billable_sms == 0