async aggregate(tasks) {
    const results = await series(tasks)

    return results.reduce(
      (aggregate, result) => ({
        stdout: String(aggregate.stdout) + String(result.stdout),
        stderr: String(aggregate.stderr) + String(result.stderr),
        children: [...aggregate.children, result.child],
      }),
      {
        stdout: '',
        stderr: '',
        children: [],
      },
    )
  }
Aggregate some exec tasks.

@private
@param {Promise.<ExecResult>[]} tasks An array of tasks
@returns {MultipleExecResult}
@throws {ExecError}