Robustify time measurement test

* Reduce influence of measurement pro-/epilogue by running the critical
  section multiple times
* Keep test time constant by decreasing iterations for long sleep tests
* Increse valid sleep measurement interval to cope with the increased
  runtime due to stack protection/redzone checks.

Change-Id: I4331afe4b56b2b50d909b311a47f56ed16c53d10