Test semantics for Yosys cells more exhaustively

[chameco]

Right now, we only have ad-hoc testing of the semantics of some Yosys cells in the integration test suite. It would not be an especially heavy lift to do some randomized testing, i.e.:

• generate random input bitvectors for the input ports of the cell
• produce a simple JSON Yosys netgraph containing only one cell of that type
• load the JSON into Yosys and simulate it on the generated inputs
• load the JSON into SAWScript and call the resulting function term on the generated inputs
• check that all of the outputs match