Fail nicer on missing expected_result in test.toml files

[jjcnn]

Description

If the test.toml file in a test does not contain the entries expected_result and expected_result_new_encoding, the end-to-end tests fail with an unhelpful error message.

This PR improves the error message.

Checklist

• I have linked to any relevant issues.
• I have commented my code, particularly in hard-to-understand areas.
• I have updated the documentation where relevant (API docs, the reference, and the Sway book).
  • If my change requires substantial documentation changes, I have requested support from the DevRel team
• I have added tests that prove my fix is effective or that my feature works.
• I have added (or requested a maintainer to add) the necessary Breaking* or New Feature labels where relevant.
• I have done my best to ensure that my PR adheres to the Fuel Labs Code Review Standards.
• I have requested a review from the relevant team or maintainers.