Use NSTimeInterval for Async Verifiers

Currently timeouts get casted to an integer, so timeouts like 0.1 will not work. Especially considering the code in KWProbePoller is built to probe every 0.1 seconds and runs on NSTimeInterval already, I think this is just a straight up bug.

The practical result of this is that test suites with lots of async work end up running for significantly longer than they need to.

It looks like it might be a regression from behavior fixed before here: https://github.com/allending/Kiwi/commit/b39fe450ba0d6f2433303e257f97259d39a167c0

I didn't see any existing tests that seemed exceptionally relevant, so I didn't add any. Existing tests still pass.