There are several validations now enforced, such as:
# The description should be larger than x length
# The main page should exist
# The main page should be unique per newsletter
# The newsletter name should be unique
How to write the test:
# First, take a look at the behavior while creating a newsletter at `Special:CreateNewsletter`
# Take a look at the function being called on clicking submit button - and in this case its https://github.com/wikimedia/mediawiki-extensions-Newsletter/blob/master/includes/specials/SpecialNewsletterCreate.php#L74
# Then take a look at https://gerrit.wikimedia.org/r/#/c/399995/3/tests/specials/SpecialNewsletterCreateTest.php and see how exactly we are testing it.
# Understand what validation you are going to test, and write a new function in the file `SpecialNewsletterCreateTest.php`with a new payload `($input)` that can mock it. For eg: with a faulty description or bad main page, etc. For debugging, you can always use `var_dump( $res ); ` as in the example gerrit change line 31
# Execute all the tests, make sure things are green, and push.