Support running tests in parallel that use the smoke test framework

[carolynvs]

The smoke test framework in tests/tester use things like os.SetEnv and os.Chdir that prevent us from safely running these tests in parallel. Let's get creative about how we are running things (like using porter build -f -d instead of changing into the directory), so that setting these isn't necessary. Or setting env vars on the tester, and it will pass that into porter when it runs the command, etc.