Use fake timers in tests

[dnlup]

Sometimes timer-based tests fail. Maybe using a fake timer would solve this.

[dnlup]

I am having trouble with the implementation of the refresh() function of timers. It seems it's not working properly, or I might have missed something in its usage.

Ref: sinonjs/fake-timers#187

Anyway, it seems like they still need to publish the proposed fix.

[dnlup]

We would not need this anymore if #56 is resolved.