Add a settings test

[ewallah]

Plugins can have defective settings.

It is possible to detect some errors by running the blocks/admin_bookmarks behat checks:

- php /home/travis/build/moodle/vendor/bin/behat --config /home/travis/build/moodledata/behat_moodledata/behatrun/behat/behat.yml --format progress --tags="@block_admin_bookmarks"

The admin_bookmarks block is usually the first plugin that complains about incorrect or not saved settings. When everything is OK this extra test passes.

Perhaps this can be the base for an extra check?