testcase: give more time for instance destruction.

During teardown, we sometimes have errors that processes are still running after being stopped, something like:

Fault: <Fault 91: 'STILL_RUNNING: TestTelegraf-0'>

This is because we only retried slapos node report two times, but slapos node report tells supervisor to stop processes without waiting and when processes takes time to stop, this is sometimes not enough. Increase to retry up to 10 times by default.