*Don't* silence the deprecation warnings in test

While working on #2784, I noticed that we basically turned off deprecation warnings in test mode (which traces back to very first public commit). I don't think this is a good idea as this is where we are most likely to see it, and in general we should try to deal with those as soon as possible.

So I enabled it again. There's only one warning left, which turned out to be a real bug :smile: (See the commit message for details.)