Allow self-referencing of course nodes in expert rules to check for test results, attempts and dates

In the past it was not possible to e.g. display a test only when not yet done or hide the group enrollment when the user did already enroll. This was due to cyclic dependencies that have been detected. But actually, dependencies to itself should not be a problem, thus they should not be included to this cyclic test.

This issue will allow referencing in the following functions:

• getOnyxTestOutcomeZK
• getOnyxTestOutcome
• getAttempts
• getInitialEnrollmentDate
• getLastAttemptDate
• getRecentEnrollmentDate
• getPassed
• getScore