Test class comment genration from various .md files

[Bajger]

Changed test class comment generation either from instructions.md & introduction.md or description.md.
This is needed for geneerating exercise test classes in Pharo from problem specifications that don't specify description.md, but instead they use introduction.md and instructions.md.

Example using description.md: https://github.com/exercism/problem-specifications/tree/main/exercises/darts
Example using introduction.md and instructions.md: https://github.com/exercism/problem-specifications/tree/main/exercises/gigasecond

[github-actions]

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