Integration tests + CI : CMake -> pytest

This branch reimplements Batsim integration tests with pytest instead of CMake.
This should permit to finally use Meson instead of CMake!

Modification overview.

• Reimplement tests with pytest.
• Provide a release.nix to build Batsim, its doxygen doc, its integration tests.
• Use release.nix in the CI script. CI can now be blazing fast: https://framagit.org/batsim/batsim/-/jobs/401010

Pros of using pytest over CMake.

• Maintainability++. @cmommess will no longer have "CMake is terrible" excuse to not write tests!
• Fixtures are amazing.
• Mature, convenient and powerful runtime engine.
• HTML reports are convenient to inspect an instance that failed.
• Integration tests can be fully separated from the Batsim binary. This permits a very clean test workflow.
• Tests can be skipped or marked as expected to fail.
  • Permits to keep instances known to fail in the test suite and to follow their evolution.

Things lost in the process.

• Tests cannot be executed in parallel for now. @mmercier suggested to use unix sockets in the output directory of each test to fix this.
• Unittests are no longer executed. Current unittests are very insufficient, ugly and should be rewritten — this will be for another issue/MR.