improve function/throttle spec, change time/now to allow mocking it + improve mockRandom

@satazor said he was having issues with `function/throttle` not calling the function at the "tail" of the interval.

I added more tests and added a way to mock the `time/now` to make sure behaviour is consistent but I could not replicate any bugs - after mocking the `Date.now` to sync with the `jasmine.Clock` all tests passes...

Expected behavior:

```
||||||||||||||||||||||||| (pause) |||||||||||||||||||||||||
X    X    X    X    X    X        X    X    X    X    X    X
```
`|` is a throttled-function call and `X` is the actual `fn` execution:

Let me know if anyone have any idea on how to improve the tests.

PS: @satazor issue was with a window scroll handler, not sure if that is the cause of the issue (maybe the scroll event messes up with the `setTimeout` and `Date.now` - blocks the thread and somehow skips execution...)