To clarify,
Qantas Cadets and Jetstar Cadets are not employed by either airline or the group. They have signed a contract but it is merely a set of rules (don't be stupid, turn up to classes, don't turn up drunk, respect everyone etc. etc.). The job at the end is merely a promise dependent on economic conditions and personnel performance.
Therefore they can't join the union as far as I understand.
The testing for the Jetstar Cadetship is largely handled by the training organisations, for Qantas it is handled in house using the same process (although as someone already said possibly different weighting) as the DE guys minus the sim.