add db mock

packages/hydrooj/src/service/__mocks__/db.ts

@@ -0,0 +1,31 @@
+/* eslint-disable import/no-mutable-exports */
+import { EventEmitter } from 'events';
+import mongodb, { Db, MongoClient } from 'mongodb';
+
+export let db: Db = null;
+export let client: MongoClient = null;
+
+export const bus = new EventEmitter();
+
+mongodb.MongoClient.connect(
+    // @ts-ignore
+    global.__MONGO_URI__,
+    { useNewUrlParser: true, useUnifiedTopology: true },
+).then((c) => {
+    // @ts-ignore
+    db = c.db(global.__MONGO_DB_NAME__);
+    client = c;
+    bus.emit('connect');
+});
+
+export function collection(c: string) {
+    return db.collection(c);
+}
+
+export function getClient() {
+    return client;
+}
+
+export function getDb() {
+    return db;
+}