Run most `core::num` tests in const context too
This adds some infrastructure for something I was going to use in #131566, but it felt worthwhile enough on its own to merge/discuss separately.
Essentially, right now we tend to rely on UI tests to ensure that things work in const context, rather than just using library tests. This uses a few simple macro tricks to make it *relatively* painless to execute tests in both runtime and compile-time context. And this only applies to the numeric tests, and not anything else.
Recommended to review without whitespace in the diff.
cc `@RalfJung`
38eaf608eb