tests: implement sync_with_i3 and use it instead of sleep()

Also use open_standard_window() in a few more places where appropriate

testcases/t/70-force_focus_wrapping.t

@@ -73,6 +73,9 @@ cmd 'layout tabbed';
 cmd 'focus parent';
 
 $third = open_standard_window($x);
+
+sync_with_i3($x);
+
 is($x->input_focus, $third->id, 'third window focused');
 
 cmd 'focus left';