This patch (originally by Achim Gratz) adds a mechanism for generating 
packages marked as 'test' as described in [1].

I'm not committed to any of the details, but I would like to get 
something with this functionality in, so tell me how you'd like it 
implemented and I'll do it...

[1] https://cygwin.com/ml/cygwin-apps/2016-12/msg00005.html