In Cargo Extension, a lot of classes do not have PHPUnit tests.
This task requires you to write PHPUnit tests for a class which does not have PHPUnit test.
You SHOULD refer to https://www.mediawiki.org/wiki/Manual:PHP_unit_testing/Writing_unit_tests , which will help you to write tests.
For reference, you should check PHPUnit tests written for MediaWiki core - you can find it in the directory tests/phpunit/includes/ .
You are expected to provide a patch in Wikimedia Gerrit. See https://www.mediawiki.org/wiki/Gerrit/Tutorial for how to set up Git and Gerrit.