Bus assertempty

[jasonmccreary]

Similar to the MailFake, this adds a unified assertion to test nothing was placed on the bus - either dispatched, chained, or batched.

Before

Bus::fake()

// ...

Bus::assertNothingDispatched();
Bus::assertNothingBatched();

After

Bus::fake()

// ...

Bus::assertNothingPlaced();

This also adds assertNothingChained which is simply an alias for assertNothingDispatched. However, it provides some symmetry with the other assertions and lowers to barrier to entry. Otherwise the developer needs to know that a job chain immediately dispatches the first job and therefore they can use assertNothingDispatched.

[jasonmccreary]

@taylorotwell, please feel free to wordsmith the name. Since assertNothingDispatched was already taken and does something else, I was having trouble thinking of other words in the Bus vocabulary to rely the method intent.