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
When CDC ACM is used as stdio the first thread in the test may have a
different ID than #3. The test code will now look at the printed thread
information (id, prio) as they are created. This avoids the need for a
table with ID/prio.
