RFC: Set the default log level to INFO

[Wilfred]

If I have a test that's emitting a bunch of warnings, it's really useful to see them. I expected that this crate would enable that, but it only seems to log errors (i.e. tracing::error!("foo") but not tracing::warn!("foo")).

I can see them if I set RUST_LOG=warn, but it took me a while to figure it out, and this crate is so close to Just Working out of the box :)

I see this has been requested in #25, but only a per-test setting was added. If it was a crate feature (as the author initially proposed), I'd use it, but maybe test-log needs a higher default log level? This seems to be a common gotcha (e.g. #23 too).

[d-e-s-o]

Yes, I think it makes sense to change, given that output is captured by default anyway. Feel free to open a pull request.