Allow the "recorded at" time to be asserted for events.

It may be useful to be able to assert the time at which a specific event has been recorded, specifically when the event is expected to be fired at some specific time as part of a process that uses timeout messages.

Perhaps we could do this by adding "functional options" to the ToRecordEvent[Type]() functions, something like the following:

test.Expect(
    AdvanceTime(ByDuration(24 * time.Hour)),
    ToRecordEvent(
        MyCoolEvent{ ... },
        RecordedAt(someTime),
    ),
)

We'd need to think carefully about whether the option types passed to ToRecordEvent() should be the same type as those passed to ToRecordEventType() or not, and if not, design the an option interface so that options like RecordedAt() can be passed to both assertions.

Start by reviewing the ToRecordEvent and ToRecordEventType assertion APIs mentioned in the issue, along with how AdvanceTime and timeout messages are handled. Define the option design for RecordedAt and verify that both event assertions can check an expected recorded time; done means tests can assert a specific event timestamp.