messageBox.t: alternative approach for X11

Avoid using Perl exit() in after callback
See #14

t/messageBox.t

@@ -19,10 +19,13 @@ my $top = MainWindow->new(-title => 'MessageBox Test');
 $top->after(1000, 
  sub{
  if( $top->windowingsystem eq 'x11' ){
- ok(1); 
- exit(); # No using $top->destroy here, because we get grab error messages on X11
- # But if we use this on windows, we get crashes.
-
+ # No using $top->destroy here, because we get grab error messages on X11
+ my $msgbox = $top->interp->widget(
+ '.__tk__messagebox', # implementation detail; see msgbox.tcl
+ 'Tcl::pTk::Toplevel',
+ );
+ $msgbox->focus;
+ $msgbox->eventGenerate('<Escape>');
  }
  else{
  $top->destroy(); # This works without errors on windows