Long-term test enumeration support

[epage]

13 will see us use dirty hacks but what we really need is a feature built-in to the compiler

See https://internals.rust-lang.org/t/from-life-before-main-to-common-life-in-main/16006/25?u=dtolnay

[epage]

Notes

• davidhewitt, daboross expressed interest in writing an RFC for #[distributed_slice]