Merge #7619

7619: Add #[track_caller] to assist tests r=matklad a=yoshuawuyts

This points the source of a failed assertion to the code which called it, rather than the location within the assertion helper method. While working on https://github.com/rust-analyzer/rust-analyzer/pull/7617 I had trouble locating some failing tests, and it was only by adding these attributes during development that I was able to locate them.

This is only applied to test helpers, which means it comes at no runtime cost. And even then: I didn't experience any noticeable performance with this enabled or disabled. Mostly just a more pleasant experience debugging test failures (:

Co-authored-by: Yoshua Wuyts <yoshuawuyts@gmail.com>