Function run 

pub fn run(
    plan: &Plan,
    fallback_cwd: &Path,
    child_output: ChildOutput,
    events: &mut dyn FnMut(Event),
) -> Result<u8, EngineError>

Really run it: inherit stdio, wait for it to finish, hand back its exit code verbatim.

fallback_cwd is used when the plan names no directory of its own.

The return value is not Result<()>: “the tests failed” and “pmpx failed” are two different things, and pmpx test has to pass a nonzero exit code through to the caller’s script – that is the only meaningful contract of this command.