Rollup merge of #139511 - Zalathar:panic-payload, r=Mark-Simulacrum

libtest: Pass the test's panic payload as Option instead of Result

Passing a `Result<(), &dyn Any>` to `calc_result` requires awkward code at both call sites to build a fake result, for no real benefit. It's much easier to just pass the payload as `Option<&dyn Any>`.

No functional change.

Author: ChrisDenton