Improvements to governance tests

[smelc]

Changelog

- description: |
    Shorten and modernize some governance tests
# uncomment types applicable to the change:
  type:
  # - feature        # introduces a new feature
  # - breaking       # the API has changed in a breaking way
  # - compatible     # the API has changed but is non-breaking
  # - optimisation   # measurable performance improvements
  # - improvement    # QoL changes e.g. refactoring
  # - bugfix         # fixes a defect
  - test           # fixes/modifies tests
  # - maintenance    # not directly related to the code
  # - release        # related to a new release preparation
  # - documentation  # change in code docs, haddocks...

Context

This started out with me working on #706 and on the way I saw multiple ways to improve the existing code. This PR was born.

Checklist

• Commit sequence broadly makes sense and commits have useful messages
• New tests are added if needed and existing tests are updated. See Running tests for more details
• Self-reviewed the diff

[smelc]

Note that removing the type parameter also aligns with how this kind of type is usually written, see e.g. https://github.com/IntersectMBO/cardano-api/blob/3bbdf8ae962d91602a4d8b0b416e90a0e8024943/cardano-api/internal/Cardano/Api/DeserialiseAnyOf.hs#L237