[PATCH cygport] Add a command to make a test release

[Andrew Schulman]

> 
> 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

Cygport needs a way to specify which versions are prev, curr, and test in
cygport files. David Rothenberger and I each proposed a method [1,2]. It
doesn't matter much to me which method is picked, but it's definitely a
missing feature.

[1]https://sourceware.org/ml/cygwin-apps/2013-06/msg00032.html
[2]https://sourceware.org/ml/cygwin-apps/2015-06/msg00038.html