Fix the thumbnail-verification Test

The third parameter of the thumnail_verify() function had been updated to
const GLocalFileStat, so update the thumbnail-verification test likewise
so that the test works properly on all supported platforms.

https://bugzilla.gnome.org/show_bug.cgi?id=711547

gio/tests/thumbnail-verification.c

@@ -91,7 +91,7 @@ test_validity (void)
   /* Run all the tests. */
   for (i = 0; i < G_N_ELEMENTS (tests); i++)
     {
-      GStatBuf stat_buf;
+      GLocalFileStat stat_buf;
       const gchar *thumbnail_path;
       gchar *file_uri;
       gboolean result;