In MediaWiki core, the ApiDisabled and ApiQueryDisabled classes are modules that just spit out "This module is disabled". Regardless, we should still have tests for this functionality.
If you're not already familiar with PHPUnit tests, https://www.mediawiki.org/wiki/Manual:PHP_unit_testing/Writing_unit_tests has links to some good material on the basics of testing.
I would also recommend looking at other tests in the tests/phpunit/includes/api directory of MediaWiki core for an example of how we currently test API modules. Since these modules aren't used by default, you'll need to register them with $wgAPIModules and one of the API query module types to be able to test them.
I'll mentor this for Google-Code-in-2017