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/events/keyboard.cpp

@@ -27,6 +27,10 @@
 #include "wx/uiaction.h"
 #include "wx/vector.h"
 
+#ifdef __WXGTK__
+#include "wx/stopwatch.h"
+#endif
+
 namespace
 {
 
@@ -229,7 +233,11 @@ void KeyboardEventTestCase::setUp()
     m_win = new KeyboardTestWindow(wxTheApp->GetTopWindow());
     wxYield();
     m_win->SetFocus();
-    wxYield(); // needed to show the new window
+
+#ifdef __WXGTK__
+    for ( wxStopWatch sw; sw.Time() < 10; )
+#endif
+        wxYield(); // needed to show the new window
 
     // The window might get some key up events when it's being shown if the key
     // was pressed when the program was started and released after the window