Replace usages of assertFileOccurences by usage of new functions ✨ (4/n)

[smelc]

Changelog

- description: |
    Replace usages of assertFileOccurences by usage of new functions 
# 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

• Contributes to fixing assertFileOccurences doesn't check CBOR payload #415
• Contrary to previous PRs, this one doesn't introduce golden files: instead it makes some existing tests that have nondeterministic content more precise.
• I did that, because - during the previous PRs -, I witnessed some tests that could be enhanced.

How to trust this PR

• The new assert* functions try to be helpful and provide good error messages
• The new assert* functions are specialized to JSON string->string mappings. That is intended, to make their usage easy and the tests easy to read.

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]

Thanks @carbolymer, change done in all functions 👍