test:crit: do cleanup before generating new images

Signed-off-by: Ruslan Kuprieiev <kupruser@gmail.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>

test/crit/Makefile

-images:
+images: clean
 	setsid ./loop.sh < /dev/null &> /dev/null & \
 	../../criu dump -v4 -o dump.log -D ./ -t $${!} --shell-job