Add logging to the integration test

[tomaka]

cc #1323

Should help understand what's going on.

[tomaka]

I think it's the first time I re-run CI because it's green and I want it to be red.

[romanb]

Using RUST_LOG=debug may give more insight, because the curious thing is that when the example fails it is due to 10 minutes without output, supposedly, yet the query timeout is set to 5 minutes. If the query timeout isn't utterly broken, this may even mean that a code path somewhere outside libp2p-kad stalls / hangs. I've also never been able to reproduce such a 10 minute hang running the example locally, which may furthermore indicate that the environment in which the example is run in CI is another contributing factor.

[romanb]

Superseded by #1493.