Additional tests in CI

[Jeremy Drake]

On Fri, 13 Jun 2025, Jeremy Drake via Cygwin-developers wrote:

> I got permission denied trying to push to a new branch.
> I'm working on posix_spawn tests on github, since that's where the actions
> would run, and also locally and on Linux.
>
> https://github.com/jeremyd2019/cygwin-stc/compare/main...refs/heads/posix-spawn

Oh, also let me know if you think this sort of test belongs somewhere else
(maybe winsup/testsuite? though I don't know how it would work out there)