Consider swapping to `Backtrace::new_unresolved`

`Backtrace::force_capture` (which is what we do know) is quite expensive computation-wise, we should try out: https://docs.rs/backtrace/latest/backtrace/struct.Backtrace.html#method.new_unresolved, and see how it performs.

Start by locating the existing Backtrace::force_capture call in the Rust code and read the backtrace crate documentation for Backtrace::new_unresolved. Compare their performance in the relevant concurrent-testing path, then determine whether the swap is suitable and verify that existing behavior remains acceptable.

Written by the indexing model from the issue text.