Testers are chosen subjects to go into a containment chamber and interact with the ULCs. It is normal for subjects to end up dying or becoming incapacitated, depending on the ULC.
If the ULC is peaceful, then no lives are at risk.
The levels of the testers are gained by surviving interactions and nothing else.
The higher level they reach, the bigger their payments will be.
Also, if proven to be effective in combat and containment of ULCs, they could eventually become Guards.