Refactor code waiting for events in the test suite

We already had WX_ASSERT_EVENT_OCCURS_IN macro and a recent commit also
added code doing almost the same thing manually in wxTopLevelWindow unit
test, which was one version too many.

Replace both of them with the new EventCounter::WaitEvent().

No real changes, this is just a refactoring.

tests/controls/windowtest.cpp

@@ -152,7 +152,7 @@ void WindowTestCase::FocusEvent()
 
     m_window->SetFocus();
 
-    WX_ASSERT_EVENT_OCCURS_IN(setfocus, 1, 500);
+    CPPUNIT_ASSERT(setfocus.WaitEvent(500));
     CPPUNIT_ASSERT(m_window->HasFocus());
 
     wxButton* button = new wxButton(wxTheApp->GetTopWindow(), wxID_ANY);