Test DateScheduler functionality for MainScheduler
Test DateScheduler functions on QueueScheduler
Add interval-based scheduleAfter to TestScheduler
Add default leeway of 0 to TestScheduler

The leeway isn’t really used in testing anyways.
Test TestScheduler advancement and dequeuing
Verify TestScheduler.currentDate as well
Merge branch 'swift-development' into scheduler-tests
++ is a thing, certainly
Merge pull request #1492 from ReactiveCocoa/scheduler-tests

Scheduler tests