Of course you should never ask to be subpoenaed. You inform the Plaintiff that "I would never would willingly testify about all the illegal things I saw at form

Of course you should never ask to be subpoenaed. You inform the Plaintiff that "I would never would willingly testify about all the illegal things I saw at former employer, you'd have to subpoena me to get me to talk!". You can then answer that question honestly with "No, I told them I'd never talk, but they subpoenaed me and forced me to."