gloo-timers: don't consume callback in Interval closure

[samcday]

Otherwise Interval only works on first callback and fails after that

[OddCoincidence]

Could we also add a test that triggers this bug?

[samcday]

@OddCoincidence yep working on tests now. As it happens integration tests are not running in CI, which is why it's reporting that these changes are okay here, when in fact fixing the bug actually causes the interval_cancel test to fail.

[samcday]

@OddCoincidence alright I wrangled the tests. Hope you don't mind but I did a little bit of re-jiggery once I understood how they worked to simplify them a little.

The interval test now ensures that 5 ticks of the interval pass. interval_stream was already doing that so now they're consistent. I removed the Cell stuff in favour of just collect()ing the Stream and counting how many results we got.

The interval_cancel test started failing once I fixed the original bug. I reworked it a little to avoid the dance it was doing in favour of just combining a Timeout stream with the Interval stream we're examining.

[fitzgen]

Looks great with one nitpick to fix before we merge this; thanks @samcday!

[samcday]

Sweet, feedback addressed. Looks like CI is running headless tests now too, nice!

[fitzgen]

Thanks!