Work around a crash in wxGTK3 GUI test suite

The tests crashed after running ModalDialogsTestCase::FileDialog() because the
next call to wxYield() resulted in the completion of an async operation
launched by the native file dialog which tried to dereference the already
destroyed dialog.

tests/controls/dialogtest.cpp

@@ -79,6 +79,13 @@ void ModalDialogsTestCase::FileDialog()
     CPPUNIT_ASSERT_EQUAL((int)wxID_OK, rc);
 
     CPPUNIT_ASSERT_EQUAL("test.txt", dlg.GetFilename());
+
+#ifdef __WXGTK3__
+    // The native file dialog in GTK+ 3 launches an async operation which tries
+    // to dereference the already deleted dialog object if we don't let it to
+    // complete before leaving this function.
+    wxYield();
+#endif
 }