tests: fix outdated periodic_discovery rule fixture

The update tests inject rules into the base site via the REST API. Since
werk 22104 the endpoint rejects a value_raw whose migrated form differs
from the input. The fixture omitted severity_changed_service_params,
which the valuespec transform adds, so rule creation failed with 400.

Change-Id: I3c58de2938b46616b77333f60291d0d46c5f1dcc