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/testableframe.h

@@ -35,6 +35,13 @@ public:
     int GetCount() { return m_frame->GetEventCount(m_type); }
     void Clear() { m_frame->ClearEventCount(m_type); }
 
+    // Sometimes we need to yield a few times before getting the event we
+    // expect, so provide a function waiting for the expected event for up to
+    // the given number of milliseconds (supposed to be divisible by 50).
+    //
+    // Return true if we did receive the event or false otherwise.
+    bool WaitEvent(int timeInMs = 1000);
+
 private:
     wxEventType m_type;
     wxTestableFrame* m_frame;