Refactored CI infrastructure.

[p-costa]

This PR features a major change in the CI testing infrastructure, designed by Louis Henry @lohenry-escience in the framework of the ExaFLOW project.

The new infrastructure:

• Makes it easy to test different environments and tests automatically NVIDIA and Intel, in addition to GNU.
• Organizes tests in matrices per compiler vendor.
• Creates a sequential dependency between "compiling" and "running" workflows, using the executable generated during compilation for the running step. The "running" test is not triggered if the "compiling" test fails.
• Allows automatic tests with different compiling profiles (e.g., debug and optimized builds).
• Avoids long log outputs. Only a subset of the log is printed, and the full logs may be visualized in a separate step, or as a downloadable artifact.

[lohenry-escience]

Do you prefer putting the test-related scripts in the .github repository, rather than in a dedicated scripts/testing directory which may be more accessible to people that could use them as reference? I do not have a strong feeling.

[p-costa]

Yes... My logic is that the user who runs the test locally will run it in their own already set environment, which is
not necessarily the same as that is used on the CI. In that sense, the set-up of the environment is CI-specific.

[p-costa]

@loostrum, to follow up on a query over Zulip, do you have an opinion about this? Thanks!