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.

/reviewed-on nexedi/slapos.core!178