feature request: support async function when mocking return

[marcelomorgado]

Is your feature request related to a problem? Please describe.
I was not able to pass an async function in order to return a dynamic value, only when the function is sync it works fine.

Describe the solution you'd like

myFake.myFunction.returns(async () => { ... })

[kbrizzle]

+1

[arjun-io]

I can confirm this doesn't work, if you modify the async test case here: https://github.com/defi-wonderland/smock/blob/main/test/unit/programmable-function-logic/type-handling.spec.ts#L237 to something like

it('should wait for result as promise', async () => {
  const asyncRequest = (n: number) => {
    return new Promise(function (resolve) {
      setTimeout(() => {
        resolve(n * 10);
      }, 10)
    });
  };
  fake.getInputtedUint256.returns(async (number: number) => asyncRequest(number));

  expect(await fake.callStatic.getInputtedUint256(10)).to.equal(100);
});

it will fail. I believe this is an issue with performing an async action within RxJS's .subscribe but I'm not sure what the right fix is.

[robercano]

I created some tests some time ago to also demonstrate the issue. You can find them here: #143