ctest: change API of when to skip

[tgross35]

Currently our config looks a bit like this:

    cfg.skip_field(move |struct_, field| {
        (struct_ == "siginfo_t" && field == "_pad") ||
        (musl && struct_ == "glob_t" && field == "gl_flags") ||
        // ...
    });

A problem is that we have no validation that e.g. _pad actually exists in siginfo_t. It would be better to have something like the following:

cfg.skip_field(&[
    // format: `(struct, field, extra_condition)`
    ("siginfo_t", "_pad", true),
    ("glob_t", "gl_flags", musl),
])

This would skip if any any of the entries match struct and field, and the condition for that row is true. However, it could also warn if any of those items are never used meaning the struct or field don't exist.

[tgross35]

For the ctest rewrite it would also be nice to be able to specify volatile_item with an annotation rather than in the build script.