Fix regression in test_langs.php

Commit 78cee358 added file Web.config to plugins and lang folder.
test_langs.php treated the file as a plugin folder and a language file.
Merge branch 'fix13-test_langs' of github.com:atrol/mantisbt into atrol-fix13-test_langs
Fix #17510: regression in test_langs.php
The helper_alternate_colors function used to add odd/even classes to HTML
elements, but dhx worked to remove these in 1.3.

Related functions were already removed, however this function was missed.