Function download_test 

pub async fn download_test(
    client: &Client,
    server: &Server,
    single: bool,
    progress: Arc<SpeedProgress>,
) -> Result<(f64, f64, u64, Vec<f64>), SpeedtestError>

Run download bandwidth test against the given server.

Returns (avg_speed_bps, peak_speed_bps, total_bytes_downloaded, speed_samples).

Errors

Returns SpeedtestError::NetworkError if all download streams fail. Returns SpeedtestError::Context if the server URL is invalid.