ctest: change API of when to skip

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.

Start from the existing ctest skip_field API and the configuration call sites shown in the issue. Define the replacement entry format and ensure unused struct/field entries can be warned about; done means matching conditional entries are skipped and missing entries are reported.