Implement Verilator testbench for CV32E20

[DBees]

This is a nice-to-have, not must-have, feature.

Within CORE-V-VERIF there is a simple test-bench called the "core testbench" that can be run with Verilator. This testbench can only support simple testcases with no asynchronous exceptions and there is no checking against a reference model. Nevertheless it is handy as a demonstration platform and can recreate some bugs that are identified in the UVM environment.

Currently the "core testbench" is only working for the CV32E40P.

This task is complete when a "core testbench" is in place and running under Verilator for the CV32E20. It should be capable of running any test-program that does not create asynchronous exceptions.