Add "Test settings" to filter management page

The management page shows filters turned on as active without checking whether the filter functionalities are or actually can be in effect.
Some filters are depending on external programs (HTML tidy, Tex,...) and these external programs need to be set up outside of Moodle setup. They are also not mentioned in the server "environment" page.

In case the external helper programs are not set up or not set up properly, then the filters are actually not in effect while still showing active status as "On". This results in confusion and extra difficulties when troubleshooting filter-related problems.