Adding `bless` support

[phansch]

Would there be any reason not to include the bless support from the rustc version of compiletest?

This seems to be useful for both miri and Clippy:

• Allow blessing test stdout/stderr rust-lang/miri#1588
• [Roadmap] Turn cargo dev into Clippys ./x.py rust-lang/rust-clippy#5394

I'd be happy to try and copy the feature over, if there's nothing speaking against that.

[Manishearth]

I'd be happy to accept it.

Ideally we can uplift libtest and libcompiletest out of tree into crates so that clippy can just use the same one rustc does

[Munksgaard]

Closed by #231