Make wxUIActionSimulator works more reliably on GTK/X11

Add wxXSync class which adds delays between the synthesized events
sufficient for them to be processed by the higher layers (X, GTK).

This makes it possible to run the UI tests by default with wxGTK.

Closes https://github.com/wxWidgets/wxWidgets/pull/1845

tests/controls/windowtest.cpp

@@ -27,6 +27,7 @@
 #include "wx/caret.h"
 #include "wx/cshelp.h"
 #include "wx/scopedptr.h"
+#include "wx/stopwatch.h"
 #include "wx/tooltip.h"
 
 class WindowTestCase
@@ -35,6 +36,13 @@ public:
     WindowTestCase()
         : m_window(new wxWindow(wxTheApp->GetTopWindow(), wxID_ANY))
     {
+    #ifdef __WXGTK3__
+        // Without this, when running this test suite solo it succeeds,
+        // but not when running it together with the other tests !!
+        // Not needed when run under Xvfb display.
+        for ( wxStopWatch sw; sw.Time() < 50; )
+            wxYield();
+    #endif
     }
 
     ~WindowTestCase()