Change test that was failing due to new Marshmallow behaviour

Boolean fields in marshmallow have various values that get changed to
True or False. The value 'Yes' now gets changed to True,  which was
causing a test to start failing. We could change the schemas to stop
'Yes' from being changed to True, but the data for boolean fields comes
from admin, where it is only allowed to have certain values anyway so
this just fixes the test.

tests/app/service/test_rest.py

@@ -887,7 +887,7 @@ def test_update_service_sets_volumes(
 @pytest.mark.parametrize('value, expected_status, expected_persisted', (
     (True, 200, True),
     (False, 200, False),
-    ('Yes', 400, None),
+    ('unknown', 400, None),
 ))
 def test_update_service_sets_research_consent(
     admin_request,