Additional tests in CI

[Jon Turney]

As an experiment, I've added another job to the CI which is run after 
every push to the Cygwin repo, which clones from the repo [1] and runs 
those tests against the just-built Cygwin.

Please, feel free to push additional tests there, (ideally after or 
simultaneously with a fix, so we don't get failures :))

(or to suggest additional regression tests, if you don't have push rights)

Results are reported to the #cygwin-developers IRC channel.

[1] https://cygwin.com/cgit/cygwin-apps/stc/