Finally nailed (permanently, I hope) two loader issues.  First, and most annoyingly, TestLoader.discover() alters sys.path, so I'm working around it by making and restoring a copy of sys.path.  Second, for some odd reason importing doesn't work using /private/var/... as a path (where the tmp dir is on os x), but works fine if you use /var/... (which is symlinked to /private/var/...)