function advanceTestQueue() {
    if (!config.blocking && !config.queue.length && config.depth === 0) {
      done();
      return;
    }

    var testTasks = config.queue.shift();
    addToTaskQueue(testTasks());

    if (priorityCount > 0) {
      priorityCount--;
    }

    advance();
  }
Advance the testQueue to the next test to process. Call done() if testQueue completes.