Tests for the computation of blockchain time.

The BlockchainTime in consensus used to be ubiquitous throughout the code base, but is now only used in one place: when we are checking if we should produce a block. It is a simple abstraction that returns the current slot number, if it is known (it might be unknown of the current ledger state is too far behind the wallclock). In addition to the problem of the current slot being unknown, it must also deal with discontinuities in the system's wallclock: NTP might adjust the clock forward or backward, or, worse, the user might change their wallclock by a large amount. We don't try to deal with all of these cases:

• if the clock jumps forward (so we "skip slots") this is no problem
• if the clock is moved back a small amount so that we are still in the same slot when we expected to be in the next, also okay
• if the clock is moved back by more than that, so that the current slot would actually decrease, we throw an exception; it's then up to the user (or the wallet) to restart the node.

Since all our tests run in an IO simulator, we can test this by having the clock behave very erratically. We then compute (in a model) what we expect the behaviour of the BlockchainTime to be given a specific erratic behaviour, and then verify that it matches the model.