tests: explicitly set input hint

Since i3 honors the “Globally Active Input” focus model, we need to
explicitly state that we are not using that in our testcases :).

This requires X11::XCB from git to work (commit
71b25dcaafc509e710b8fd7de20c97ac3549fc39).

testcases/lib/i3test.pm

@@ -337,6 +337,7 @@ sub open_window {
     $args{name} //= 'Window ' . counter_window();
 
     my $window = $x->root->create_child(%args);
+    $window->add_hint('input');
 
     if ($before_map) {
         # TODO: investigate why _create is not needed