I ran into this problem when trying to run simpletests with coder installed. I got this error:
<?phpFatal error: require_once() [function.require]: Failed opening required 'Text/Diff.php' (include_path='.:/usr/share/php:/usr/share/pear:sites/all/modules/coder/scripts/coder_format/tests') in /var/www/test_memetracker/sites/all/modules/coder/scripts/coder_format/tests/all.test on line 6?>
I eventually found that line 4 in all.tests had the wrong path. It's missing the second tests directory. This is what I changed things so it works.
<?phpset_include_path(get_include_path() . PATH_SEPARATOR . drupal_get_path('module', 'coder') .'/scripts/coder_format/tests/tests');?>