Add Unit Tests for AppealsApi AMS Serializers

[stevenjcumming]

Summary

• Add more test examples for missing attributes
• Update the spec to test against the JSON:API specifications

Related issue(s)

• https://github.com/department-of-veterans-affairs/va.gov-team/issues/83613

Testing done

• [x] Add more test examples for missing attributes

Screenshots

Acceptance criteria

• [x] unit test coverage is 100%

Requested Feedback

Should this example test the strings are in the right format (e.g., ISO 8601)

expect(rendered_hash[:updated_at].to_s).to eq evidence_submission.updated_at.to_s