Add extra test to Lasagna

[Callisto13]

The exercise can be completed without doing it properly, this extra test ensures against hardcoded values in the solution.

[github-actions]

Thank you for contributing to exercism/elixir 💜 🎉. This is an automated PR comment 🤖 for the maintainers of this repository that helps with the PR review process. You can safely ignore it and wait for a maintainer to review your changes.

Based on the files changed in this PR, it would be good to pay attention to the following details when reviewing the PR:

• General steps

  • 🏆 Does this PR need to receive a label with a reputation modifier (x:size/{tiny,small,medium,large,massive})? (A medium reputation amount is awarded by default, see docs)

• Any exercise changed

  • 👤 Does the author of the PR need to be added as an author or contributor in <exercise>/.meta/config.json (see docs)?
  • 🔬 Do the analyzer and the analyzer comments exist for this exercise? Do they need to be changed?
  • 📜 Does the design file (<exercise>/.meta/design.md) need to be updated to document new implementation decisions?

• Concept exercise changed

  • 🌲 Do prerequisites and practices in config.json need to be updated?
  • 📖 Does the concept introduction provide all necessary information to solve this exercise?

• Concept exercise tests changed

  • ⚪️ Are all tests un-skipped?
  • 🔢 Are all tests annotated with @tag task_id?
  • 🐈 Can all tests be understood by reading the test's block only (e.g. no module attributes, no setup functions)?

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Automated comment created by PR Commenter 🤖.

[angelikatyborska]

Thank you!