This would be a parent task for writing unit tests to check DB operations executed by extension:Newsleter. It would be very similar to testAddSubscriber in https://github.com/wikimedia/mediawiki-extensions-Newsletter/blob/master/tests/NewsletterDbTest.php and mostly go into the same file as well.

The operations are mainly functions in https://github.com/wikimedia/mediawiki-extensions-Newsletter/blob/master/includes/NewsletterDb.php which needs to be tested.

To write the *unit* test:

1. Add your function with correct name (as to what task you are solving) to NewsletterDbTest.php
2. Write what the Expected Db operations are - follow something like the $mockWriteDb operations in testAddSubscriber at https://github.com/wikimedia/mediawiki-extensions-Newsletter/blob/master/tests/NewsletterDbTest.php
3. Install phpmyadmin(https://help.ubuntu.com/lts/serverguide/phpmyadmin.html) to even see what all tables newsletter extension use. These are prefixed with nl_tablename
4. Assert if things are correct - look at last line of testAddSubscriber
5. Run all the tests once by executing tests/phpunit/phpunit.php extensions/Newsletter from core/

*Expected know-how*: A bit of PHP, some idea about unit testing - and that would be it.
*Difficulty*: moderate, but you can always see what was done before and copy.