Autotest: use _exit() instead of exit() to simulate an unclean exit

We're not going to crash to avoid core dumps getting recorded by
systemd, filling up filesystems or showing dialog boxes. We just need to
exit without running destructors. Now, exit() isn't supposed to run
function-scope destructors, so just in case we use _exit().

Change-Id: I0b48fc8e90304e0dacc3fffd14e909ff248ce1a1
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@qt.io>