In MediaWiki core, the ComposerInstalled (includes/libs/composer/ComposerInstalled.php) class does not have any tests.
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/libs/composer directory of MediaWiki core for an example of how we currently test the other libs/composer classes. The test will probably be similar to the already existing ComposerLockTest.
I'll mentor this for Google-Code-in-2017