Migrate tests from doctests to #[test]s

[nightkr]

Doctests require a separate compilation per test, which is slow and uses immense amounts of RAM if allowed to run in parallel.

[NickLarsenNZ]

Could you please provide an example of moving a doctest to a #[test]?

[NickLarsenNZ]

Ah, I see. There are some ignored doc-tests.

I quite like the doctests. Or rather that knowing that an example is correct as the code changes.
An alternative could be to move things out to examples.