Figure out how to write unit tests for the sim.v files.

We want a structured way to write unit tests for the sim.v files (and a way to run them).

We want the unit tests to be compatible with at a minimum;
* Yosys
* Verilator
* Icarus Verilog

But making it compatible with commercial simulators would be good too.

I *think* we want to use [Cocotb](http://cocotb.readthedocs.io/en/latest/introduction.html) but I have never used it myself.