Pass-thru std{out,err} while also collecting it

[killercup]

Not a pressing issue at all, just a random thing I wanted recently for nicer test debugging.

[pksunkara]

It was a bit difficult to get this setup in rust compared to other languages, but I have something similar here, https://github.com/pksunkara/cargo-workspaces/blob/master/src/utils/cargo.rs#L26

[killercup]

Yeah that is similar to what I had in mind.

PS: This link to your code works