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/webtest.cpp

@@ -72,7 +72,7 @@ private:
 };
 
 //Convenience macro
-#define ENSURE_LOADED WX_ASSERT_EVENT_OCCURS_IN((*m_loaded), 1, 1000)
+#define ENSURE_LOADED CHECK( m_loaded->WaitEvent() )
 
 // register in the unnamed registry so that these tests are run by default
 CPPUNIT_TEST_SUITE_REGISTRATION( WebTestCase );