[C2] Adaptive test in learning path returns status to "not attempted"

[ywarnier]

This is related to #3657 and the testing procedure is described at the end there.
Basically, when finishing an adaptive test, the status should always be set to "completed" and never return to "not attempted", as the point of the adpative test is just to finish on the result screen, not to get a particular score.

It's important because it could lock the student progress if the learning path is set to use requirements.