ServiceRestartTest: make it more resilient

The test spawned new processes to run services in, but didn't guard test
suite with due attention not to leak those processes into test
environment.

Proper guarding mechanism is already implemented for another test,
ServiceLauncherTest. Hence it was moved to their common ancestor
(ServiceTestBase) and reused in both test classes.

Closes-Bug: #1382573
Change-Id: If4ee7ed09696c3f5f3273f4d280c567b328ca75c
Merge "ServiceRestartTest: make it more resilient"