Hi Chris.
Today we had python sprint and I've guessed to Anton to refactor the patch in good way with properly setting default values from regrtest.main to argparse args. Then use proper argparse actions for manipulating that args.

After all we can use Namespace object returned from argparse.parse() or argparse.parse_known_args() if needed as input for next processing.

It will be big enough patch but I like to move it forward after double checking.

regrtestlib.py name is good to me.