Since SLAAC now happens on the up event, the tests needs to ensure, that
`gnrc_ipv6_nib_iface_up()` is called before assuming SLAAC happened. For
the mock interface, this is done by gnrc_ipv6_nib_iface_up().
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
