Add support for inputing via stdin with run-make-support
This PR adds the facility to set a input bytes that will be passed via the standard input.
This is useful for testing `rustc -` (and soon `rustdoc -`).
In #124611 took the approach of having a dedicated `run` method but it is not very convenient to use and would necessitate many functions, one for success, one for fail, ...
Instead this PR takes a different approach and allows setting the input bytes as if it were a parameter and when calling the (now custom) `output` function, we write the input bytes into stdin. I think this gives us maximum flexibility in the implementation and a simple interface for users.
To test this new logic I ported `tests/run-make/stdin-non-utf8/` to an `rmake.rs` one.
r? `@jieyouxu`
c27d3d5d2a