If I testify in court can an insurance company come after me for a fraud that was committed over 5 years ago

Generally speaking, the insurance company would not be in a position to pursue criminal fraud charges - that is the domain of the state. The insurance company will sometimes report suspected fraud, and will cooperate in the investigation, but they have no role in prosecuting the crime.

If you received payment on a claim as a result of the fraud, however, that is a lightly different question - can the insurance company come after you to recover what they paid you back when the fraud was commited?

Often times a criminal conviction will also involve an order of restitution to pay actual damages to the victims of the crime. In this instance, the insurance company would be the victim, so if resitution is ordered, then they have their recovery (in theory).

If not, the insurance co may be able to pursue damages from you (unless a statute of limitations bars recovery)