MDL-45296 tests: ensure all submissions exist

MDL-45296 tests: ensure all submissions exist

The changes performed with the change from
create_from_user() to create_from_submission()
in practice enforce a new restriction about
submissions having to exist in order to fire
their lock/unlock events.

This did not exist before the change and, also,
it seems that the assign api itself, submission->lock() ...
also accepts non existing submissions.

So I was not 100% sure about how to fix these events:

1) enforcing submission to exist.
2) firing them only if the submission exist.

I've gone with 1) for now, making tests to pass. But will
raise the question in the Tracker, just in case we have
to move to the 2) approach for any reason.