remove pytest.mark.usefixtures decorator

while it's nice to use the decorator to signify fixtures with side
effects, it has unfortunate problems of completely overriding any
fixtures you've declared in the funcargs - so isn't really suitable
for our usecase where we often have other fixtures we rely on to
return values to us.

So for consistency, let's remove this and stick to using funcargs
to define our fixtures

tests/app/db.py

@@ -10,8 +10,8 @@ def create_user(mobile_number="+447700900986", email="notify@digital.cabinet-off
         'mobile_number': mobile_number,
         'state': 'active'
     }
-    usr = User.query.filter_by(email_address=email).first()
-    if not usr:
-        usr = User(**data)
-    save_model_user(usr)
-    return usr
+    user = User.query.filter_by(email_address=email).first()
+    if not user:
+        user = User(**data)
+    save_model_user(user)
+    return user