Retrieve the running assessment test sessions after the coach stopped an
assessment mode. The process is slightly delayed.

Don't archive test in a course element if the test entry is already
deleted, same for survey and self test

The NodeEvaluation makes it difficult to use a course node in the
learning path access concept. So it is refactored to a minimum extent,
so that it can covering the previous functionality.
As a replacement a new security callback and a new tree node type is
introduced.