Remove old test_ function for testing markdown code

[filipsch]

The old markdown testing functions are used in only two courses:

• Reporting with RMarkdown (GitHub, Teach, Campus)
• Communicating with Data in the TidyVerse (GitHub, Teach, Campus)

In short:

• [ ] rewrite SCTs to use the new markdown functions that are check_ based.
• [ ] Get rid of a lot of test_-based markdown testing code in testwhat.

Classic technical debt resolution, so not urgent, but can bite us in the future.