Tell them to subpoena you - then the choice to testify is not yours any more. You'll have to. But you can tell future employers that you were forced, if you'd like to.
This is smart. Guilty as sin or not, testifying against an employer seems risky for one's career. You want to be able to explain to future employers what happened in a manner that takes personal character assessment out of the equation: somebody broke the law and you got a subpoena. EDIT: I've worked at some shops that were "cultish" and would look at testifying without a subpoena as not being part of the team. Fucke…
You can choose not to be a part of these organizations.