rna-transcription: Add generators and regenerate tests

[tasxatzial]

Second time i'm updating this. In the first update there used to be some kind of error checking in the tests that wasn't present in the specs. I updated the tests, now it's a good time to simplify the example solution as well.

[github-actions]

This PR touches files which potentially affect the outcome of the tests of an exercise. This will cause all students' solutions to affected exercises to be re-tested.

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