Add testbenches or formal verification for each IP

[phsauter]

This is a 'best possible' kind of task and not a fixed required goal.
Ideally all simple IPs should have formal verification, all CDCs should have timed testbenches (maybe even with synthesis to an open technology and netlist simulation) and all other IPs should have a reasonably exhaustive testbench that covers all specified behavior including edge cases.

As much as possible should run with open-source simulators but it is not a requirement.