`copilot-core`: Increase test coverage about copilot HOT 6 CLOSED

Change Manager: Confirmed that the issue exists.

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Technical Lead: Confirmed that the issue should be addressed.

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Technical Lead: Issue scheduled for fixing in Copilot 3.19.

Fix assigned to: @ivanperez-keera .

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Implementor: Solution implemented, review requested.

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Change Manager: Verified that:

• Solution is implemented:
  • The code proposed compiles and passes all tests. Details:
    Build log: https://app.travis-ci.com/github/Copilot-Language/copilot/builds/269344778
  • The solution proposed produces the expected result. Details:
    Evaluated by manual inspection. Running tests with coverage enabled achieves coverage of 97% for top-level definitions (37 out of 38 definitions). The one not tested is uTypeType, which cannot be tested due to being part of an existential type (it's necessity is being discussed in #484). The modules Copilot.Core.Operators and Copilot.Core.Spec and Copilot.Core.Expr only contain datatype definitions, so they do not need to be tested.
• Implementation is documented. Details:
  New tests are documented.
• Change history is clear.
• Commit messages are clear.
• Changelogs are updated.
• Examples are updated. Details:
  No updates needed.
• Required version bumps are evaluated. Details:
  Bump not needed. Changes affect tests only.

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Change Manager: Implementation ready to be merged.

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