[RFC] CMake: Capture stderr when running tests.

A possible fix for #1301, based on @Islah's last comment there.
Should we flip this to WIP until it gets figured out?

Personally, I'm not sure how I feel about erroring out if things are detected on stderr.  I could see unit tests that want to test some logic and provide the correct error, but it happens in a way that the message is emitted on stderr (perhaps because it's meant to be invoked early on).  I wouldn't call such a thing a problem.  So if I had to lean a direction, it would be to capture and emit them.  I'd rather busted did the capturing and emit the stderr if the test failed though.  But given that busted seems to be getting relatively little love lately, us capturing it and emitting it later seems like the best alternative.