accessibility-dump: Print diff by default

There's just no reason not to; it makes things less tedious to
run from the command line.

testsuite/a11y/accessibility-dump.c

@@ -734,7 +734,7 @@ test_ui_file (GFile *file)
 
       if (diff && diff[0])
         {
-          g_test_message ("Contents don't match expected contents:\n%s", diff);
+          g_printerr ("Contents don't match expected contents:\n%s", diff);
           g_test_fail ();
           g_free (diff);
         }