Firebase cloud function unit test creates actual real document in firestore. How can I disable that behaviour for tests?

I have written a function, that does create a document in firestore. Very basic snippet of the function:

  await db.collection(VERFIY_CODES_PATH).doc().set({
    user: uid,
    code: result,
  });

The problem is when I run my tests, this function actually creates real firestore documents. Is there a way of disabling that? My test looks like this:

import "dotenv/config";
import * as functions from "firebase-functions-test";
import * as path from "path";
import { anyContext, emptyData, emptyContext, anyUsername } from "./helpers/testConstants";
import { https } from "firebase-functions/v1";
import Message from "../src/types/message";
import getError from "./helpers/helperFunctions";

const projectConfig = {
  projectId: "flowus-app-dev",
  databaseURL: process.env.DATABASE_URL,
};
const testEnv = functions(projectConfig, path.resolve("./flowus-app-dev-fb-admin-sdk-key.json"));

import { verify } from "../src";

describe('Testing "verification"', () => {
  it("test verification function works", async () => {
    const data = {
      locale: "en",
      userName: anyUsername,
    };
    const result: Message = await testEnv.wrap(verify)(data, anyContext);
    expect(result.code).toBe(200);
    expect(result.message.length).toBe(6);
  });
});

Any Ideas on how to fix this issue ?