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
While looking at tests/gnrc_ipv6_ext_frag again while writing
RIOT-OS/Release-Specs#137, I noticed that several of tests that I
definitely wrote myself from scretch are attributed wrong (and
sometimes even documented wrong). I guess this was caused by just
copy-pasting the files...
