Add details of how to debug tests locally
Problem/Motivation
When testing locally with gitlab-ci-local, dump() calls in the module code don't work.
This is because the module gets copied to gitlab-ci-local/artifacts/composer/. Presumably, you'd have to redo the `composer` step. Or maybe commit the dump() calls to git -- I don't know.
Anyway, it would be useful to document that after the composer step has been done, that's where the active module code lives and that's where debugging can be done.