virtual_time schedules equal entries FIFO

Added a counter that is used to track the insertion order of scheduled tasks. I only did this for virtual_time, but if you think it looks good and works as expected I assume that the same needs to be done for the "real" schedulers. I also think it would be wise to extract this functionality to its own class. But I thought this could be a good first step to take so that you can look at it and see if you think it will work out.

I had to switch the order in some of the tests as well, but the way I see it these are now appearing in, what I consider, the natural order.