Add tests for modulepath service registration

[tlinkowski]

Currently, our specs only test if a service is properly registered on classpath ("proper service is registered on classpath"). It's easy to check, because we're testing on classpath only (due to Groovy 2). However, we should find some way to also test if a service is properly registered on modulepath (and generally test everything on modulepath too - should be possible with Groovy 3).