Emit stderr from tests when running Tor in more cases.

[nmathewson]

When Tor is running as a daemon, its stderr is only used for real errors that we should probably be paying attention to. But there are still cases (see https://github.com/torproject/stem/issues/52#issuecomment-577932788) where we suppress Tor's stderr logging. This made it hard to diagnose https://trac.torproject.org/projects/tor/ticket/33006 .

I'll write a patch when I can.

[atagar]

Thanks Nick! A patch is certainly welcome. Or, alternatively, if you'd like for me to write it I'd be willing to now that you've expressed that having stderr in these specific tests is particularly valuable to you.