CI refactoring

[Youssef1313]

Currently, our CI is a simple single stage.

For Windows, we make an official build and pack, and use the produced packages for integration tests.

For non-Windows, we make an unofficial build and pack, and use those packages for integration tests.

We should only build and pack once, on Windows, and use the same artifacts for all jobs, because at the end of the day, these are the packages that we ship.

Detailed proposal

• Introduce a "Build and pack" stage. This stage will run on Windows only. It will only build production projects, and won't build any test projects.
• The packages produced during that stage are published to artifacts
• Introduce 6 test stages (the combination of OSes (Windows,Mac,Linux) and Configuration (Debug/Release)).
  • Each stage will download the packages.
  • Build the test projects (may potentially build parts of products (again), esp for unit tests)
  • Run the tests

[nohwnd]

👍 agree, this is how we build vstest (afaik)