doctests show a full stack trace of the panic despite `RUST_BACKTRACE` not being set

[Lokathor]

at least since yesterday, when i get a panic in a doc test it shows a full backtrace of the panic. Over a page long, full of the old-style useless stack frames info, not the newer info that's usually nicer by cutting out the stuff burred inside the standard library.

however, normal tests using the #[test] attribute give the expected output of just saying "FAILED" and then printing the panic message (no stack trace) after all the test results.

rustc 1.64.0-nightly (4d6d601c8 2022-07-26)