I am no breeder, also no tester, but will try to comment anyway.
I think the breeder will look for testers with distinct characteristics, with some minimum experience growing (normally).
Distinct growing methods, general environment differences are important too (temperature, lights, humidity, etc).
The tester shoud be able to comunicate well, report frequently if possible with good key pics.
The idea is for the tester to be able to open a window for the breeder, so he can look how his strain goes at his environment/style. Some times the breeder will ask for specific things, and be able to provide the breeder this open window with good comunication and information is the key to be a good tester imho.
It is not easy, someday I will love to do it, I just need one more grow for some sedative strain and then I will be up for grabs if anyone want it, as I grow mostly for my wife and we have enough weed for years, all vaccum sealed at the freezer