Fuzz LEC flow

Our LEC flow has light usage and no fuzzing which means it probably has bitrotted some. To keep it fresh and bugfree we should fuzz it. Potential options of increasing complexity:

* prove unopt IR equivalent opt IR
* prove opt IR equivalent to parsed netlist (requires a backend flow such as openroad)
* do per-stage LEC'ing of a pipeline

An interesting issue with LEC is that it may timeout. This doesn't pose a fundamental problem as timeout can be considered a non-failing sample.