使用 Jest 进行单元测试时出现如下问题：
Jest did not exit one second after the test run has completed.
This usually means that there are asynchronous operations that weren't stopped in your tests. Consider running Jest with `--detectOpenHandles` to troubleshoot this issue.可能原因：测试时有连接数据库，测试结束没有关闭。
解决办法：
···
import * as mongoose from 'mongoose';
···
describe('AppController (e2e)', () => {
···
afterAll(async () => {
mongoose.disconnect();
});
})