Add option to make verbose = true apply recursively to nested test sets

[mhauru]

I wanted a feature where verbose = true would apply to nested test sets all the way down the nesting, so I made it. Any interest in having this as part of DefaultTestSet? I didn't discuss this with anyone before hand, so "we don't want this, please close" is a valid response.

I wasn't sure about the best interface. I went with verbose = 2, but happy to consider e.g. verbose = :recursive. Currently verbose = 1 is equivalent to verbose = true.

I'm leaving this as a draft because it currently breaks the DefaultTestSet constructors: If someone uses the "full" inner constructor with a Bool for verbose it'll error. DefaultTestSet isn't exported, but I would still like to fix that. I'll pause here to get comments though before implementing the fix.