Test libstd backtrace type

[RalfJung]

@Aaron1011 added support for capturing backtraces in Miri, but so far we only test directly using the Miri API for that. Once actually using the libstd types works, we should test those as well.

@Aaron1011 what is the status of that?

[Aaron1011]

I think we need to bump the backtrace-rs submodule in libstd.

[Aaron1011]

This is being done in rust-lang/rust#77731