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/broadcast_message/test_rest.py

@@ -268,7 +268,7 @@ def test_update_broadcast_message_status_doesnt_let_you_update_other_things(admi
 def test_update_broadcast_message_status_stores_cancelled_by_and_cancelled_at(admin_request, sample_service):
     t = create_template(sample_service, BROADCAST_TYPE)
     bm = create_broadcast_message(t, status=BroadcastStatusType.BROADCASTING)
-    canceller = create_user('canceller@gov.uk')
+    canceller = create_user(email='canceller@gov.uk')
 
     response = admin_request.post(
         'broadcast_message.update_broadcast_message_status',
@@ -290,7 +290,7 @@ def test_update_broadcast_message_status_stores_approved_by_and_approved_at_and_
 ):
     t = create_template(sample_service, BROADCAST_TYPE)
     bm = create_broadcast_message(t, status=BroadcastStatusType.PENDING_APPROVAL)
-    approver = create_user('approver@gov.uk')
+    approver = create_user(email='approver@gov.uk')
     mock_task = mocker.patch('app.celery.broadcast_message_tasks.send_broadcast_message.apply_async')
 
     response = admin_request.post(
@@ -305,3 +305,24 @@ def test_update_broadcast_message_status_stores_approved_by_and_approved_at_and_
     assert response['approved_at'] is not None
     assert response['approved_by_id'] == str(approver.id)
     mock_task.assert_called_once_with(kwargs={'broadcast_message_id': str(bm.id)}, queue='notify-internal-tasks')
+
+
+def test_update_broadcast_message_status_rejects_approval_from_creator(
+    admin_request,
+    sample_service,
+    mocker
+):
+    t = create_template(sample_service, BROADCAST_TYPE)
+    bm = create_broadcast_message(t, status=BroadcastStatusType.PENDING_APPROVAL)
+    mock_task = mocker.patch('app.celery.broadcast_message_tasks.send_broadcast_message.apply_async')
+
+    response = admin_request.post(
+        'broadcast_message.update_broadcast_message_status',
+        _data={'status': BroadcastStatusType.BROADCASTING, 'created_by': str(t.created_by_id)},
+        service_id=t.service_id,
+        broadcast_message_id=bm.id,
+        _expected_status=400
+    )
+
+    assert mock_task.called is False
+    assert f'cannot approve their own broadcast' in response['message']

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',