Add the "core" testbench to cv32e20-dv

[MikeOpenHWGroup]

The purpose of the "core" testbench is twofold:

• a simple testbench that runs under Verilator.
• support for the ACT4.0 testsuite.

For the most part, all files in this PR are independent of the UVM testbench. The sole exception is mk/Common.mk. In that file I have made edits to "clean up" the code, mostly for readability and usability.