tests: latency_measure: Update interrupt latency

Updates the interrupt to thread benchmark tests to address
a number of items.
  1. Updates descriptions to correctly indicate that it is not
     interrupt latency being measured, but the time to leave
     the interrupt and return to a thread.
  2. Repeats the test numerous times instead of just once to
     get an average.
  3. Overhead from obtaining timestamps is now accounted for.
  4. Adds support for returning to a user thread.

Signed-off-by: Peter Mitsis <peter.mitsis@intel.com>