Auto merge of #127633 - SamuelMarks:eq-exit-code, r=dtolnay

[library/std/src/process.rs] `PartialEq` for `ExitCode`

Converting a third-party CLI to a library so started passing around [`std::process::ExitCode`](https://doc.rust-lang.org/std/process/struct.ExitCode.html) in an `Either`. Then I realised the tests can't be modified to compare equality of `ExitCode`s.

This PR fixes this oversight.

Author: bors