[valid-expect-in-promise] Enforce for Observable .subscribe() also

If you use an RxJS Observable and put an expect in the subscribe callback, you run into the same issue as with promises:

it('passes', () => {
  someObservable.subscribe(() => {
    expect(false).toBe(true);
  });
});

Can valid-expect-in-promise be extended or duplicated to check for subscribe?

[SimenB]

Right, thanks.

PR welcome! 😀

[SimenB]

I like the idea! It might make more sense as a separate rule? Not sure, maybe not 😃