Add tests for shortcomings of associated type bounds
Adds the test in https://github.com/rust-lang/rust/pull/122791#issuecomment-2011433015
Turns out that #121123 is what breaks `tests/ui/associated-type-bounds/cant-see-copy-bound-from-child-rigid.rs` (passes on nightly), but given that associated type bounds haven't landed anywhere yet, I'm happy with breaking it.
This is unrelated to #122791, which just needed that original commit e6b64c61941120f734657106ae2479d05b463197 stacked on top of it so that it wouldn't have tests failing.
r? lcnr
08ac38b661