Fix infinite loop in piglit_is_extension_in_string.
If piglit_is_extension_in_string() finds an extension whose name begins with the name it is searching for, but is not equal to the name it is searching for (e.g. it is searching for EXT_transform_feedback but it finds EXT_transform_feedback2), we need it to advance to the end of the substring it's found before restarting the search, otherwise it will loop indefinitely. Prevents a hang when running some transform feedback tests on nVidia's proprietary Linux driver. Reviewed-by: Brian Paul <email@example.com>