service_test: Test stopping services after migration to another cgroup

The root cause of "After OTA, device is not responding" is that
Service::Stop() did not stop services that had been migrated to another
v2 cgroup. Add a unit test for that scenario.

Bug: 308900853
Change-Id: I590fde90e31326290c2c8ce8fce51e94b215a979
Signed-off-by: Bart Van Assche <bvanassche@google.com>