Regression Improvements

Created by: davidharrishmc

regression-wally --nightly is now running all sims including short buildroot in Verilator, VCS, and Questa

Verilator is more than twice as fast as the commercial sims with our present optimization flags, so regression-wally is switched to run Verilator as the default sim. This also makes Wally less reliant on commercial tools.

Some cleanup of testbench-fp, but not yet running in Verilator.

Some steps toward running VCS in lockstep.