[rami3l]

Cherry-picked from #5194, this patch unifies the testing experience of regular subprocesses and ParkedChild by making the latter return the same Assert type upon finish as the former.

In other words, this allows returning code assertions and stdout/stderr snapshot testing on ParkedChild as one would normally do with regular subprocesses.

https://github.com/rust-lang/rustup/pull/5194/changes#diff-c6c864ad9d6d36c77a5ba7036d549e1c24a55bea1002c562f98e574db19240e9 gives an example that demonstrates how this can be helpful:

// TODO: Two processes downloading the same file share the same `.partial` path. Whichever finishes
// first renames it to its final name, and the other one then fails to find it.
//
// See: <https://github.com/rust-lang/rustup/issues/4607>
#[tokio::test]
async fn concurrent_download_of_same_file_fails() {
    let cx = CliTestContext::new(Scenario::SimpleV2).await;

    let parked = cx.spawn_at(
        CHECKPOINT_DOWNLOAD_BEFORE_RENAME,
        ["rustup", "toolchain", "install", "nightly"],
    );
    cx.config
        .expect(["rustup", "toolchain", "install", "nightly"])
        .await
        .is_ok();

    parked
        .resume()
        .with_stderr(snapbox::str![[r#"
...
error: component download failed for [..]: could not rename 'downloaded' file from '[..].partial' to '[..]':[..]
...
"#]])
        .is_err();
}