I think testing is different for us and CG,
According to a Software Engineering book by Ian Sommerville, "Testing is intended to show that a program does what it is intended to do and to discover program defects before it is put into use. "
Basically, is C3PO working as intended? Does the game crash when he uses his basic or special? Does it freeze? Is he correctly granting one stack of translation per special ability? Is confuse debuff working as it should? Is translation applying the stacking benefits? does the game freeze when someone gains a translation after having a maximum of 4 stacks?
and so on... I think in that regard, C3PO was tested fine. I haven't seen or encountered any issues using him in a team. Haven't seen anyone else point out issues with his kit either.
Given how limited manpower and time they had, I understand they were unable to test C3PO in raids or pair him up with zFinn (although a very very obvious combination)
I think the testing isn't perfect by any means such as Bastilla not applying Tenacity correctly when she was released, and they shouldn't recommend squad synergies unless they've tested the unit with multiple team compositions by theorycrafting (which sorta implies they had time to test C3PO under zFinn lead)