How to unit test a cross-contract call using NEAR (AssemblyScript)?

I'm trying to follow this example on how to create a unit test for a cross-contract call.

I have a cross-contract call like this

// Class with cross-contract calls
export class CalculatorApi {
  add(a: string, b: string): ContractPromise {
    let args: AddArgs = { a, b };
    let promise = ContractPromise.create(OTHER_CONTRACT, "addLongNumbers", args.encode(), 100000000000000);
    return promise;
  }
}


// function to unit test
export function calculate(a: string , b: string): void { // Need to return void according to current spec
  let calculator = new CalculatorApi();
  let promise = calculator.add(a, b);
  promise.returnAsResult();
}

You may notice this function returns void, which is a bit confusing because the contract is returning a promise. This is because it's calling a function elsewhere and the compiler thinks it's void.

(In future releases this will be changed.)

And a test like this

// VS Code complains about the async word
it("add one digit", async function() {
  const params = { a: "1", b: "99" };
  const result = await contract.calculate(params);
  expect(result).toBe("100");
});

However, the above test complains about the syntax with the following error:

An async function or method must return a 'Promise'. Make sure you have a declaration for 'Promise' or include 'ES2015' in your '--lib' option.ts(2697)

I'm not sure, but maybe it has something to do with my tsconfig.json file, which looks like this:

{
  "extends": "../node_modules/assemblyscript/std/assembly.json",
  "include": ["./**/*.ts"]
}

I tried to add the following, but it didn't work

  "compilerOptions": {
    "lib": ["es2020"]
  }