rez-test PKG arg default to current pwd's package.py

A feature to be able to give a rez-test the ability to identify that it's being run in a src dir, and to grab the package name to run against from that package.py. I would recommend allowing this option to be separate from the --src feature also mentioned in the previous issue, as both ways have a use.

Start at the rez-test command entry point and review how its package argument and --src option are handled. Use the current working directory's package.py as the behavior reference. Done means rez-test can identify the package name from package.py when run in a source directory, with this option remaining separate from --src.

Written by the indexing model from the issue text.