XXX tests: sync_locale() Make sure to get info from global locale

This function is supposed to copy the global locale state to the
per-thread one, but until this commit, if the current locale being used
was the per-thread one, it would be a no-op.  This is an edge case that
rarely would occur in practice, as the point of this function is when
you are in the global locale, so wouldn't generally be called otherwise.
But it should handle the edge case properly.

locale.c

@@ -7227,7 +7227,7 @@ Perl_sync_locale(pTHX)
 
 #    elif defined(USE_POSIX_2008_LOCALE)
 
-    was_in_global = (LC_GLOBAL_LOCALE == uselocale((locale_t) 0));
+    was_in_global = (LC_GLOBAL_LOCALE == uselocale(LC_GLOBAL_LOCALE));
 
 #    else
 #      error Unexpected Configuration