RVFI Assert - Check "rvfi_intr.interrupt" count

Created by: silabs-robin

This PR adds an assertion relating to https://github.com/openhwgroup/cv32e40s/issues/355 .

Since the assert in the original issue is actually checking something unrelated, I wanted an assert that checks the actual problem that was discovered. (ps. Includes a small update to the formal setup too.) @silabs-oysteink do you agree that this assert checks the scenario from issue 355? It produces a similar case: