In MediaWiki core, the ApiDelete module allows deleting pages, is a pretty important module, and should have 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/api directory of MediaWiki core for an example of how we currently test API modules. For the purposes of this GCI task, only worry about testing deletion of normal pages, not files, which have special handling.
I'll mentor this for Google-Code-in-2017