Allow blessing test stdout/stderr

[Aaron1011]

Currently, I need to update the captured stdout/stderr for each test individually whenever it changes. It would be nice to have a ./miri test --bless mode, which would work like ./x.py test --bless

[RalfJung]

Yeah, that would definitely be nice. Unfortunately I have no idea what would be a good way to do this.

I am also not sure if it is worth sinking more effort into our compiletest-based infrastructure (the non-rustc version of that crate is barely maintained) or if we can use https://github.com/dtolnay/trybuild instead.

[oli-obk]

Another alternative is to use assert_cmd like in https://github.com/davidbarsky/tracing-tree/pull/21/files (if trybuild doesn't give us the control we ened)

[camelid]

@rustbot claim

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[saethlin]

@RalfJung I think this is done now?