Add tests to ensure that variant files work as expect:

Test the following situations:

• pixi-build-rattler-build & pixi-build-python (necessary, since rattler-build is handled differently here)
• variants.yaml and conda_build_config.yaml

I test:

• Whether the correct number of outputs are created
• If it properly triggers a rebuild when necessary

AI disclosure

• Tests were 50/50 created by codex and me. Most of the time, codex created a first version and I adapted them