Such a session would still have to be tied to a user with enough privileges to make the changes your candidates would need to prove their "skillz".
You shouldn't even want to let untrusted outsiders touch sensitive machines, even if you could monitor them in real time as they worked. Anyone who does hasn't been paying attention to their CVEs. As I write this, there have been 13 local Linux kernel vulnerabilities in just the past 3 months. (You may get a different number of results clicking that link at a later time.) For the most part, all you need to exploit these is to have the ability to run commands as a normal user on a system with the affected feature enabled.
I suggest that if you have to see if someone can do a sysadmin job, that you set up a VM, give the prospect full root permissions on it, and let them loose.
If you're trying to see if they can fix things, set up the VM in a working state, take a snapshot, purposely damage the VM, then set another snapshot before giving it to them. This will let you roll back to the damaged state for the next candidate, or roll back to the pristine state to set up a different form of damage. Some VM systems let you set up a "tree" of such scenarios, with different VMs branching off at different points from prior copies.
If you're trying to see if they can set up a particular service, install a clean copy of the OS either with the service in its default state or not installed at all. Then as before, take a snapshot before giving the prospect access to the VM.