Usando o mongodb-memory-server

database.js

const mongoose = require("mongoose");
const { MongoMemoryServer } = require("mongodb-memory-server");

let mongoServer = null;

const connect = async () => {
  mongoose.Promise = global.Promise;

  mongoServer = new MongoMemoryServer();
  const mongoUri = await mongoServer.getUri();
  await mongoose.connect(mongoUri, {
    useNewUrlParser: true,
    useUnifiedTopology: true,
    useFindAndModify: false,
    useCreateIndex: true
  });
};

const disconnect = async () => {
  await mongoose.disconnect();
  await mongoServer.stop();
};

// mongoose.connection
//   .on("open", () => console.log("Connection opened"))
//   .on("error", error => console.log(error))
//   .on("close", () => console.log("Connection closed"))
//   .on("connected", () => console.log("Database connected"))
//   .on("disconnected", () => console.log("Database disconnected"));

module.exports = { connect, disconnect };