Motivation

* `MTELGScheduledCallbackTests.testCancelAfterShutdownDoesNotCallCancellationCallbackAgain`
  is failing on iOS simulator CI. It schedules a callback with a `.milliseconds(1)`
  deadline, then shuts down, expecting shutdown to cancel the callback. On the
  simulator the 1ms timer fires before shutdown processes, so the callback executes
  instead of being cancelled.
* This is the same class of bug previously fixed for
  `testScheduledCallbackNotExecutedBeforeDeadline` in dcecf0c.

Modifications

* Change the scheduled callback deadline from `.milliseconds(1)` to `.hours(1)` in
  `testCancelAfterShutdownDoesNotCallCancellationCallbackAgain` (the observed failure).
* Speculatively apply the same fix to `testCancelExecutesCancellationCallback`,
  `testCancelAfterCancelDoesNotCallCancellationCallbackAgain`, and
  `testShutdownDoesNotCancelCancelledCallbacksAgain` which have the same pattern.
* The `EmbeddedEventLoop` and `NIOAsyncTestingEventLoop` subclasses use virtual time
  so this does not extend test duration.

Result

* Scheduled callback cancellation tests no longer depend on real-time precision and
  should not flake on slow CI runners or simulators.