Disable wxPaintEvent propagation tests for wxGTK3

They're currently broken and just always fail, it's useless to continue
running them until the issue described in the comment in this commit is
fixed.

tests/events/propagation.cpp

@@ -37,7 +37,12 @@
 //        some tests there. But this should be fixed and the tests reenabled
 //        because wxPaintEvent propagation in wxScrolledWindow is a perfect
 //        example of fragile code that could be broken under OS X.
-#ifndef __WXOSX__
+//
+// FIXME: Under GTK+ 3 the test is broken because a simple wxYield() is not
+//        enough to map the frame. It should be also fixed there by waiting for
+//        it to come up, with some timeout, but for now it always fails, so
+//        it's useless to run it.
+#if !defined(__WXOSX__) && !defined(__WXGTK3__)
     #define CAN_TEST_PAINT_EVENTS
 #endif