tests: t/166-assign.t: use wait_for_window

While it’s generally intended that wait_for_window is not called within
this testcase, in the first test instruction it was a mistake. The
window in fact gets mapped and therefore we should call wait_for_window.

testcases/t/166-assign.t

@@ -57,6 +57,7 @@ my $tmp = fresh_workspace;
 ok(@{get_ws_content($tmp)} == 0, 'no containers yet');
 
 my $window = open_special;
+wait_for_map($window);
 
 ok(@{get_ws_content($tmp)} == 1, 'special window got managed to current (random) workspace');