Rollup merge of #161443 - khyperia:abbys-tests, r=BoxyUwU

add internal DSL for testing binders

implements https://github.com/rust-lang/project-assumptions-on-binders/issues/8

apologies to the inevitable swarm of people who get pinged for this...

basically the only interesting diff is `compiler/rustc_hir_analysis/src/check/wfcheck.rs`, which actually implements the test. there's probably still features we want to add to this DSL (e.g. `RegionConstraint::AliasTyOutlivesViaEnv`), and there might be some bugs lurking, but this is at least a base to work off of. It's perma-unstable and for internal use only, so support and quality doesn't have to be incredibly high. For example, parser recovery and whatnot is just, bad, but whatever.

r? BoxyUwU