Function index_with_worker 

pub async fn index_with_worker(
    root: &Path,
    src: &Path,
    repo: &str,
    view: &str,
    filter: &IndexFilter,
    worker: Option<&Path>,
) -> Result<IndexSummary>

index_with, with the parse worker pinned rather than resolved.

worker is the gonzalo-parse-worker binary to parse through, or None to parse in-process. index_with passes whatever resolve_parse_worker finds; pinning it explicitly is what lets a test drive a known parse path.