Function run_plan 

pub fn run_plan(
    plan: &BenchPlan,
    plan_dir: &Path,
    results_path: &Path,
    options: &RunOptions,
    progress: &mut dyn FnMut(&str),
) -> Result<Results, String>

Execute plan, appending to results_path (resume: candidates already present are skipped). Writes the results file after every candidate.