HOW-TO: Test this Drupal composer script

In order to test this script, you'll need to get the entire Drupal repo and
run the tests there.

You'll find the tests under core/tests/Drupal/Tests/Composer/Generator.

You can get the full Drupal repo here:
https://www.drupal.org/project/drupal/git-instructions

You can find more information about running PHPUnit tests with Drupal here:
https://www.drupal.org/node/2116263

Each component in the Drupal\Composer\Plugin namespace has its own annotated test
group. You can use this group to run only the tests for this component. Like
this:

$ ./vendor/bin/phpunit -c core --group Metapackage