scripts/twister: Kill qemus with SIGKILL, not SIGTERM

On qemu_cortex_a53 with icount enabled, the qemu process will spin
hard at the end of its run (common with icount), but will also ignore
the SIGTERM twister is sending to terminate the (successful!) test
run.  The resulting zombie spinning processes end up accumulating and
poisoning the test run.

Just use a hard KILL instead of trying to be a good citizen (and in
lieu of fixing the upstream qemu bug).  I can't see why this should be
a problem.

Signed-off-by: Andy Ross <andyross@google.com>