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
(cherry picked from commit 3c94482a8f7ebc86866b6793ffde1a58bdd2b00d)
Merge "ServiceRestartTest: make it more resilient" into stable/juno