Function perform 

pub fn perform(
    steps: &[Step],
    run: &mut dyn FnMut(&HostCommand) -> Result<(), String>,
) -> Result<(), String>

Carries out steps in order, stopping at the first that fails, and answers why it failed in words the person can read: the command’s own when a command failed.

run runs one command; run_host is the real one, and a test passes its own.

Errors

The words of the first step that failed.