Function compare_programs 

pub fn compare_programs(
    expected_cmd: &[&str],
    actual_cmd: &[&str],
    input: &str,
) -> Result<(), CompareMismatch>

Run two programs on the same input and compare their outputs.

Any process-launch failure is converted into a synthetic output string so the caller still receives a CompareMismatch instead of a process-level error type.