CI: make and call reusable workflow for running tests

[jeremyd2019]

If this works, we could call this workflow from
git-for-windows/msys2-runtime too instead of duplicating it.

[jeremyd2019]

This works in msys2-runtime too: jeremyd2019/gfw-msys2-runtime@fa76d22 -> https://github.com/jeremyd2019/gfw-msys2-runtime/actions/runs/13998901877

[jeremyd2019]

I kind of expected this same sort of test matrix thing in git-for-windows/git, but it seems there's a somewhat similar but different matrix going on there. I wonder if they could be unified, and this callable workflow put into git-for-windows/git instead.

[dscho]

I kind of expected this same sort of test matrix thing in git-for-windows/git, but it seems there's a somewhat similar but different matrix going on there. I wonder if they could be unified, and this callable workflow put into git-for-windows/git instead.

The reason why git-for-windows/git does not have that matrix is that its main CI is derived from git/git's.

[dscho]

Thank you!