Let's say one could do what you envisioned ... test accounts used by 'applicants' to take a 'placement' quiz. How would you know who used 'test1' account and passed the 'placement' quiz? How would you send to them a 'congrats' message telling them they have qualified?
'Mega admin' if persons have real accounts to take the placement test ... not really ... but admin would have some work to do from time to time.
Actually, could expand offerings if those who do not pass the placement test are encouraged to take review/remedial courses to help them prep for the placement again. You might miss out on someone who just needs a little encouragement/push.
Could establish 2 student roles ... the default one used for placement course/quiz. The other ... a studentap role (student after placement'). Only the students who passed have their role changed to studentap so they can see/interact with courses where only studentap's can see/interact with them.
Accounts that linger ... not used any longer ... can be culled from time to time.
Above not the only way.
You have options ... just have to really think it through and might be best to set up a test system ... for un-intended consequences.
2 cent thoughts ...