added method to clear all firestore data

[ShoeBoom]

Description

Added a function to clear all firestore data between tests. Similar to https://github.com/firebase/firebase-js-sdk/blob/master/packages/testing/src/api/index.ts#L233

This makes it easier to run tests in offline mode

Code sample

const test = require("firebase-functions-test")({
	projectId: "project-id",
});

describe('test', () => {
	afterEach(async () => {
		await test.firestore.clearFirestoreData({ projectId: "project-id" });
	});
	it('test1', () => {
		// test that involes database changes
	});
	
	it('test2', () => {
		// test that involes database changes
	});
});

in order to implement tests I think I would need change:
"test": "mocha .tmp/spec/index.spec.js"
to
"test": "firebase emulators:exec 'mocha .tmp/spec/index.spec.js'"

I want to know if there is a better way to implement tests before I started.

[joehan]

This looks pretty good to me! 1 nit around the type you used for params. For testing, I think the best option is to make a new npm command, and use that to write + run tests:
"integrationTest": "firebase emulators:exec 'mocha .tmp/spec/index.spec.js'"

[joehan]

LGTM with one style nit. Thanks for the contribution! I'm planning to release a new version tomorrow to get this out.