Add another sleep to make menu unit test pass under wxGTK

At least when using GNOME, the frame containing the menu doesn't appear on
screen in time for wxUIActionSimulator to use it without this.

tests/menu/menu.cpp

@@ -499,6 +499,12 @@ void MenuTestCase::Events()
     m_frame->SetFocus();
     wxYield();
 
+#ifdef __WXGTK__
+    // This is another test which fails with wxGTK without this delay because
+    // the frame doesn't appear on screen in time.
+    wxMilliSleep(50);
+#endif // __WXGTK__
+
     wxUIActionSimulator sim;
     sim.KeyDown(WXK_F1);
     sim.KeyUp(WXK_F1);