Motivation

* `MTELGScheduledCallbackTests.testScheduledCallbackNotExecutedBeforeDeadline` is
  flakey on macOS CI. It schedules a callback with a 1ms deadline, sleeps for 1µs
  via `Task.sleep`, then asserts the callback hasn't fired. On a loaded CI runner,
  `Task.sleep` can overshoot well past 1ms, causing the callback to fire before
  the assertion.

Modifications

* Increase the scheduled callback deadline from `.milliseconds(1)` to `.hours(1)`
  in `_BaseScheduledCallbackTests`. This makes it impossible for the short sleep
  to overshoot past the deadline. The `EmbeddedEventLoop` and
  `NIOAsyncTestingEventLoop` subclasses use virtual time so this change is
  harmless to them.

Result

* The test reliably asserts that a callback is not executed before its deadline
  without depending on real-time sleep precision.