Test 222: call exit_gracefully

Long description:

Not calling exit_gracefully causes some other test to fail during a
complete_run sometimes.

To apply this patch, use:
curl http://cr.i3wm.org/patch/479/raw.patch | git am

@@ -39,4 +39,6 @@ cmd('[class="special"] resize grow height 160 px or 16 ppt');

 does_i3_live;

+exit_gracefully($pid);

+

 done_testing;