Sort skipped extensions in test summary

Sort flags are the same as for $exts_to_test (flags: SORT_REGULAR) in
main():730.

There was also a slight chance to branch into get_summary() with
(show_ext_summary: true) and count(NULL) use:  Given the call of
find_files() in main() on line 635, when there are no extensions to
skip, then the global variable $exts_skipped is not converted to an
array ($global[] = ...), that errors on count() then.

Therefore, we cast the global variable $exts_skipped to array before
sorting and counting.

Author: hakre

run-tests.php

@@ -3012,6 +3012,9 @@ function get_summary(bool $show_ext_summary): string
     $summary = '';
 
     if ($show_ext_summary) {
+        static $exts_skipped_sorted;
+        $exts_skipped = (array) $exts_skipped;
+        $exts_skipped_sorted ??= sort($exts_skipped);
         $summary .= '
 =====================================================================
 TEST RESULT SUMMARY