We should have integration test coverage for MinusX. Maybe we can use https://github.com/mikey179/vfsStream to mock the filesystem, I haven't fully investigated yet. If not, we can just a temporary directory.
If you're not familiar with PHPUnit 6, https://phpunit.de/getting-started/phpunit-6.html has a basic guide. The git repository for MinusX can be found in Gerrit as mediawiki/tools/minus-x.
Mentors: @Legoktm