feat(tests): add XEN tests

[jochem-brouwer]

🗒️ Description

This PR adds benchmark situations for the XEN mainnet contract.

🔗 Related Issues or PRs

Continuation of the now-closed PR in execution-spec-tests ethereum/execution-spec-tests#2101

✅ Checklist

Need to cleanup tests and add a test for reading existing slots, as well as writing zeros to a non-existing slot.

• All: Ran fast tox checks to avoid unnecessary CI fails, see also Code Standards and Enabling Pre-commit Checks:

  uvx tox -e static

• All: PR title adheres to the repo standard - it will be used as the squash commit message and should start type(scope):.
• All: Considered adding an entry to CHANGELOG.md.
• All: Considered updating the online docs in the ./docs/ directory.
• All: Set appropriate labels for the changes (only maintainers can apply labels).
• Tests: Ran mkdocs serve locally and verified the auto-generated docs for new tests in the Test Case Reference are correctly formatted.
• Tests: For PRs implementing a missed test case, update the post-mortem document to add an entry the list.
• Ported Tests: All converted JSON/YML tests from ethereum/tests or tests/static have been assigned @ported_from marker.

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