In MediaWiki core, the ApiClearHasMsg module doesn't have any tests. It's a fairly simple module that clears the "you have new messages" flag for users. You will need to create a ApiClearHasMsg test case that implements tests for the module.
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.
I'll mentor this for Google-Code-in-2017