test: show that staff member is able to save an incomplete flow w/agency

refine the tests to show that there is still incorrect behavior, namely
that as long as there are no template errors, the model validation
allows an incomplete EnrollmentFlow to be saved with a transit agency.

tests/pytest/core/admin/test_enrollment.py

@@ -186,6 +186,7 @@ def test_EnrollmentFlowForm_staff_member_with_transit_agency(
         self, admin_user_request, flow_admin_model, model_TransitAgency, active
     ):
         model_TransitAgency.active = active
+        model_TransitAgency.slug = "mst"  # use value that will map to existing templates
         model_TransitAgency.save()
 
         request = admin_user_request()
@@ -194,7 +195,7 @@ def test_EnrollmentFlowForm_staff_member_with_transit_agency(
         form_class = flow_admin_model.get_form(request)
 
         request.POST = dict(
-            system_name="testflow",
+            system_name="senior",  # use value that will map to existing templates
             supported_enrollment_methods=[models.EnrollmentMethods.DIGITAL, models.EnrollmentMethods.IN_PERSON],
             transit_agency=model_TransitAgency.id,
         )