Add Time Mocking

[tooolbox]

This PR adds a WithClock() option, which uses mixer/clock to simulate the passage of time. Tests can be written to, for example, verify the behavior of a given schedule.

This comes in handy for me personally as I'd like to test the interaction of a cron with my own service, over a long period of time.

[shubham1172]

+1, @robfig would you be interested in merging this?

[roynwang]

+1.
Without mockable clock unit tests are ugly.

[ShiyiWei1995]

+1. Without mockable clock unit tests are ugly.

+10086