Testing with TypeScript and TS-Jest

Hi there,

I was following the [TypeScript](https://github.com/GoogleCloudPlatform/functions-framework-nodejs/blob/master/docs/typescript.md) and [Testing](https://github.com/GoogleCloudPlatform/functions-framework-nodejs/blob/master/docs/testing-functions.md) guides but I had to work a lot in order to get the tests run with [ts-jest](https://www.npmjs.com/package/ts-jest).

## Setup

This is my setup:

`index.ts` defines the handler

```ts
import { Request, Response, http } from "@google-cloud/functions-framework";

// Register an HTTP function with the Functions Framework

export async function helloHTTPFunction(req: Request, res: Response) {
// Your code here

// Send an HTTP response
res.send("OK");
}

http("helloHTTPFunction", helloHTTPFunction);

```

`index.test.ts` defines the handler's test

```ts
import {
HttpFunction,
Request,
Response,
} from "@google-cloud/functions-framework";
// @ts-ignore
import { getFunction } from "@google-cloud/functions-framework/testing";

describe("HelloTests", () => {
beforeAll(async () => {
// load the module that defines HelloTests
await import("./index");
});

it("is testable", () => {
// get the function using the name it was registered with
const helloHTTPFunction = getFunction(
"helloHTTPFunction"
) as HttpFunction;

// a Request stub with a simple JSON payload
const req = {
body: { foo: "bar" },
} as Request;
// a Response stub that captures the sent response
const res = {
send: (result) => {
// assert the response matches the expected value
expect(result).toBe("OK");
},
} as Response;

// invoke the function
helloHTTPFunction(req, res);
});
});
```

`jest.config.ts` ts-jest config file

```ts
import type { JestConfigWithTsJest } from "ts-jest";

const config: JestConfigWithTsJest = {
preset: "ts-jest",
slowTestThreshold: 5000,
testEnvironment: "node",
silent: true,
modulePathIgnorePatterns: ["/dist/", "/node_modules/"],
testPathIgnorePatterns: [
"/src/fixtures/",
"/dist/",
"/node_modules/",
],
coveragePathIgnorePatterns: [
"/src/fixtures/",
"/dist/",
"/node_modules/",
],
};

export default config;
```

`tsconfig.json` defines TS settings

```json
{
"compilerOptions": {
"target": "ES2020",
"module": "CommonJS",
"moduleResolution": "nodenext",
"outDir": "dist",
"esModuleInterop": true,
"strict": true,
"skipLibCheck": true
}
}
```

## Environment

Node 18.12.1
TypeScript 4.9.5
jest 29.4.3
ts-jest 29.0.5
@google-cloud/functions-framework 3.1.3

OS: macOS Ventura on M1 Pro processor

## Error

I get this error when I try to import the `testing` submodule without the `// @ts-ignore` in the test file


I think it's related to [this TypeScript issue](https://github.com/microsoft/TypeScript/issues/33079#issuecomment-1426238391) and the way the package.json is declared in `@google-cloud/functions-framework` module.

The test will also fail if not using the --experimental-vm-modules whith jest, but that's because of the `await import("./index")` statement in the hook.

So you need to add this to the `package.json` scripts

`"test": "NODE_OPTIONS=--experimental-vm-modules jest"`

and then launch `npm test`.

## Workaround

Adding the `// @ts-ignore` does the trick. However, if I am not messing up with the TS settings too much, wouldn't it be nice to have a more TS-friendly testing module package declaration?

And if you think (and I might agree with you) that this issue is more a TS problem than your library's, then could you please extend the documentation by adding a "testing with typescript" section in the docs (feel free to add my workaround if deemed good enough)?