Rollup merge of #110590 - oli-obk:object_safe_assoc_types, r=jackh726

Add some tests around (lack of) object safety of associated types and consts

See https://rust-lang.zulipchat.com/#narrow/stream/144729-t-types/topic/.60where.20Self.3ASized.60.20on.20assoc.20types/near/351260928 for some discussion around why this isn't allowed.

We didn't have any tests for these, so I decided to add them now, even if we don't end up doing anything about it.

Author: JohnTitor