fix teardown-cgroup

Probably it is not important because 'test' cgroup dir for those processes
is removed but bash complains about pid list and echo fails so pid removal
does not take place.

Signed-off-by: Karol Wrona <k.wrona@samsung.com>

teardown-cgroup.sh

@@ -22,9 +22,9 @@ then
 	exit
 fi
 
-for pid in `cat $cgroupdir/$testdir/tasks`
+for pid in $(cat $cgroupdir/$testdir/tasks)
 do
-	echo $pid > $cgroupdir/tasks
+	exec echo $pid > $cgroupdir/tasks
 done
 
 rmdir $cgroupdir/$testdir