Randomising the question ordering in the case of a tie

[hgeorgsch]

Supersedes #199

Is your feature request related to a problem? Please describe.
It may be useful to start with questions with equal rating. In this case the next question will be taken from the arbitrary ordering in the DB, and students risk getting the same questions repeated.,

Describe the solution you'd like
If we could randomise the ordering of questions with equal rating difference, the student would get different questions even when they tie.

Describe alternatives you've considered
No other idea.

Additional context
The reason for the current solution is of course that the randomisation is expensive, and hence this may not be a practical solution. The issue came up from a user expecting a different behaviour, and the use case is valid enough. Very often you do not know how to rate the questions, and assigning equal ratings and wait for the system to learn from student performance is a reasomnable approach.

[hgeorgsch]

In theory, this should be soluble in SQL, combining the ideas from

1. https://www.petefreitag.com/item/466.cfm
2. https://learnsql.com/cookbook/how-to-order-by-two-columns-in-sql/
   We need to translate this into the PHP/Moodle API though.