test: unit test shows staff member cannot add EnrollmentFlow with agency

using the same `ModelForm` class that the `SortableEnrollmentFlowAdmin`
will provide to the admin interface, this test shows that if a staff
member tries to submit the form with a transit agency configured for
the flow, then there will be errors about `eligibility_api_` fields
to which the staff member doesn't even have edit access.

Author: angela-tran

Diff for: tests/pytest/core/admin/test_enrollment.py

@@ -165,3 +165,24 @@ def test_has_add_permission(
         request = admin_user_request(user_type)
 
         assert flow_admin_model.has_add_permission(request) == expected
+
+    @pytest.mark.parametrize("include_transit_agency", [True, False])
+    def test_EnrollmentFlowForm_staff_member(
+        self, admin_user_request, flow_admin_model, model_TransitAgency, include_transit_agency
+    ):
+        request = admin_user_request()
+
+        # get the Form class that's used in the admin add view as the user would see it
+        form_class = flow_admin_model.get_form(request)
+
+        request.POST = dict(
+            system_name="testflow",
+            supported_enrollment_methods=[models.EnrollmentMethods.DIGITAL, models.EnrollmentMethods.IN_PERSON],
+        )
+
+        if include_transit_agency:
+            request.POST.update({"transit_agency": model_TransitAgency.id})
+
+        form = form_class(request.POST)
+        assert not form.errors
+        assert form.is_valid()