test(dm): show full result of ps on kill fail (pingcap#5667)

ref pingcap#5089

Author: D3Hunter

dm/tests/_utils/wait_process_exit

@@ -16,5 +16,5 @@ while [ $WAIT_COUNT -lt 120 ]; do
 	((WAIT_COUNT++))
 done
 
-echo "process $process didn't exit after 120 seconds, current processlist: $(pgrep $process)"
+echo "process $process didn't exit after 120 seconds, current processlist: $(ps -ef | grep $process | grep -v 'grep')"
 exit 1