This plugin uses a special sandbox which allows it to run composer as a separate process during testing.
A fake local packagist repository is created from package definitions for the purpose of testing by using internally Satis as a library.
See the ComposerSandbox class for the details.
Just start vendor/bin/phpunit.
If you want to see the output of commands executed during functional testing use the --debug switch:
vendor/bin/phpunit --debug
It's nice to also add the --testdox switch then.