Make svc_static_route_intergration_test more robust

Add task_util::TaskSchedulerLock to stop+drain and start scheduler.
Use TaskSchedulerLock when verifying paths for service chain route.

Change-Id: I641eeaf4c3bcac7c44232c532dea07874f0e48e5
Merge "Make svc_static_route_intergration_test more robust"