Add basic tests, create / fix the schema and fix phpcs issues

Alright, all done! Please review! :)

Thanks, failing test now:

There was 1 error:
1) Drupal\Tests\commerce_custom_checkout_message\Functional\CommerceCustomCheckoutMessageGenericTest::testModuleGenericIssues
Drupal\Core\Extension\MissingDependencyException: Unable to install modules: module 'commerce_custom_checkout_message' is missing its dependency module commerce_checkout.

Alright, green now! Weird bug, probably caused by some commerce naming problems, as the submodule folder is named "checkout" but the module itself is called "commerce_checkout". Or the problem is, that our dependency relies on the submodule and therefore, the automatically generated composer.json can't resolve the main module... I don't know, but it works now with a manually created composer.json. Please review again!

Perfect!

Automatically closed - issue fixed for 2 weeks with no activity.