Expose FakeTimers.timers: [TimerID]

[nrako]

Do you want to request a feature or report a bug?
feature

What is the current behavior?
FakeTimers doesn't expose _timers making hard to test if a timer id is present (not cleared).

What is the expected behavior?
I was looking for a way to test the non-existence of a "timeout id" after a clearTimeout but I believe that the "timers" ids are not exposed.

With sinon this would be done like this:

describe('when unmounted', () => {
    it('clears timeout', () => {
      const clock = sinon.useFakeTimers()
      const timeout = wrapper.instance().timeout
      expect(timeout).toBeDefined()
      wrapper.unmount()
      expect(clock.timers).to.not.include.keys(timeout.id)
    })
})

What would you think of exposing it?

[thymikee]

I believe this could be addressed with this PR: #5165. cc @SimenB

[SimenB]

We still don't expose the underlying timers. we might want to expose the underlying clock though, not sure

[thymikee]

I think it wouldn't harm to return it. But it might be better idea to do it as a followup, when we're sure Lolex is here to stay.

[SimenB]

We keep track of the timers today, so potentially, we could expose it: https://github.com/facebook/jest/blob/74892f0f6a0abd6a4e9d5a7053fdcd22e4f6d23c/packages/jest-util/src/fake_timers.js#L80

Keeping that API after a Lolex migration is simple

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