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/spinctrltest.cpp

@@ -21,6 +21,7 @@
 #include "testableframe.h"
 #include "wx/uiaction.h"
 #include "wx/spinctrl.h"
+#include "wx/textctrl.h"
 
 class SpinCtrlTestCase : public CppUnit::TestCase
 {
@@ -245,6 +246,10 @@ void SpinCtrlTestCase::SetValueInsideEventHandler()
 #if wxUSE_UIACTIONSIMULATOR
     m_spin->Bind(wxEVT_SPINCTRL, &SpinCtrlTestCase::OnSpinSetValue, this);
 
+    // A dummy control with which we change the focus.
+    wxTextCtrl* text = new wxTextCtrl(wxTheApp->GetTopWindow(), wxID_ANY);
+    text->Move(m_spin->GetSize().x, m_spin->GetSize().y * 3);
+
     wxUIActionSimulator sim;
 
     // run multiple times to make sure there are no issues with keeping old value
@@ -258,7 +263,7 @@ void SpinCtrlTestCase::SetValueInsideEventHandler()
         sim.Text("20");
         wxYield();
 
-        wxTheApp->GetTopWindow()->SetFocus();
+        text->SetFocus();
         wxYield();
 
         CPPUNIT_ASSERT_EQUAL(32, m_spin->GetValue());