Disable unit test sporadically failing in wxGTK buildbot builds.

This failure seems to only happen on buildbot, at least it couldn't be
reproduced after 100 runs of the test locally, so it just doesn't seem
possible to debug it.

Disable it to allow detecting when other tests fail (which would be
unexpected, unlike this one).

tests/events/propagation.cpp

@@ -653,6 +653,13 @@ void EventPropagationTestCase::ContextMenuEvent()
 
     CPPUNIT_ASSERT_EQUAL( "cp", g_str );
 
+    // For some unfathomable reason the test below sporadically fails in wxGTK
+    // buildbot builds, so disable it there to avoid spurious failure reports.
+#ifdef __WXGTK__
+    if ( IsAutomaticTest() )
+        return;
+#endif // __WXGTK__
+
     // Right clicking outside the child should generate the event just in the
     // parent.
     g_str.clear();