ktest: Use make -s kernelrelease

The previous tail -1 broke with commit 7ff52571 ("kbuild: fake the
"Entering directory ..." message more simply")

Link: http://lkml.kernel.org/r/20141022194408.GA20989@pobox.suse.cz



Reported-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Michal Marek <mmarek@suse.cz>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>