ctest: add defines for specific headers and language selection

[mbyx]

Description

Allows defining values only for certain headers, as well as choosing which language to use when generating tests.

Closes #4598

Sources

Checklist

• Relevant tests in libc-test/semver have been updated
• No placeholder or unstable values like *LAST or *MAX are
  included (see #3131)
• Tested locally (cd libc-test && cargo test --target mytarget);
  especially relevant for platforms that may not be checked in CI

[mbyx]

Should we add back in the C++ testing in ctest-test or just add another smaller test to tests?

[mbyx]

FreeBSD is failing in a different way than usual. It might be because HashMap doesn't maintain order of the headers. EDIT: Apparently I can't read docs, BTreeMap also only maintains sorted order not insertion order. Fixed.

[tgross35]

Instead of keeping this as separate fields, could headers instead become Vec<(BoxStr, Vec<BoxStr>)>? Or put those two fields into a new struct.

This should also allows the same header to be used more than once with a different set of #defines. Probably not very useful for our purposes, but it does come up in C land.

[tgross35]

FreeBSD is failing in a different way than usual. It might be because HashMap doesn't maintain order of the headers. EDIT: Apparently I can't read docs, BTreeMap also only maintains sorted order not insertion order. Fixed.

For reference, this is usually where something like the indexmap crate could be used, e.g. serde has this as an option. But I don't think we really need it, the defines can just be kept in a vector if we don't need to dedupe them #4673 (comment)

[tgross35]

Awesome, thanks!