enforce kwargs when creating user in tests

spent WAY too long trying to figure out why my user wasn't being created
in tests. the user isn't created if their email already exists in the
system, but email isn't a required field when creating!

Note: I tried just removing the check to see if the user already exists,
but 16 tests try and create duplicate users. I'm of the belief we should
just fix all those tests but I didn't have the energy for it right now

tests/app/db.py

@@ -66,6 +66,7 @@ from app.models import (
 
 
 def create_user(
+    *,
     mobile_number="+447700900986",
     email="notify@digital.cabinet-office.gov.uk",
     state='active',