Reduce the number lines to output by only testing for intervals 0..15
to speed up the test.
In addition, run each test case 128 repetitions (it is still faster
than before) to give some confidence the short relative set actually
succeeded.
Replace direct accesses to sched_active_thread and sched_active_pid with
the helper functions thread_getpid() and thread_get_active(). This serves
two purposes:
1. It makes accidental writes to those variable from outside core less likely.
2. Casting off the volatile qualifier is now well contained to those two
functions
