Clean up throttle tests

Resolves #807 with the following changes:

* Added the option to configure the delta time in `Y.Lang.now`
* Used the delta to avoid using too many loops
* Removed duplicated tests (ie: checking if the throttled function is the same in many tests)
* Added test for the throttled function never called after the throttle time
* Consolidated into only one test case

/cc @sdesai