PLT-1451: Move constraint tests out of Spec.Contract

[sjoerdvisscher]

Only one test was still relevant.

Pre-submit checklist:

• Branch
  • [x] Tests are provided (if possible)
  • [x] Commit sequence broadly makes sense
  • [x] Key commits have useful messages
  • [x] Formatting, PNG optimization, etc. are updated
  • [ ] Important changes are reflected in changelog.d of the affected packages
• PR
  • [x] Self-reviewed the diff
  • [x] Useful pull request description
  • [ ] Reference the ADR in the PR and reference the PR in the ADR (if revelant)
  • [x] Reviewer requested