I don't think any company here will tell you it'd be a deal breaker. Certainly it will have an impact though. You could testify but still have a plan for controlling what comes up when people search your name. For example, by doing SEO on yourself.

Also, if the law that was broken was actually a major thing then I'd be hard pressed not to testify. It might affect your career, but only with companies that themselves have questionable ethics. You probably don't want to work for them anyway.