Sure retainers exist, but the question is whether it was actually a valid job or just part of a con. Or even if there's a distinction between the two.
And I don't think Syndrome really had testing in mind. Remember some of his quotes:
"Surprising, we must bring him back."
"You know, I went through quite a few supers to get it worthy to fight you, but man, it wasn't good enough!"
I think he honestly meant for Mr. Incredible to die. So if the entire job offer was a ruse, was he actually earning a paycheck? Or was Syndrome just giving him money as part of a larger plot?