Motivation

* `testFlatBlockingMapOnto` sets up a `flatMapBlocking` chain that dispatches
  work to `DispatchQueue.global()`, but never waits for the chain to complete
  before returning. The `defer` block calls `syncShutdownGracefully()`, which
  shuts down the event loop while the GCD dispatch is still in-flight. When
  the GCD work completes and tries to deliver its result back to the event
  loop, it finds the loop already shut down, causing a crash. On fast machines
  the GCD work completes before shutdown, but on slow iOS simulators shutdown
  wins the race consistently. This crash also kills whichever other test
  happens to be running concurrently (typically
  `testRepeatedTaskThatIsCancelledAfterRunningAtLeastTwiceNotifies`).

Modifications

* Capture the `EventLoopFuture` from the `flatMapBlocking` chain and call
  `.wait()` on it before the function returns, ensuring the GCD dispatch
  delivers its result back to the event loop before shutdown begins. Move the
  assertions from the `.whenSuccess` closure to after the `.wait()`.

Result

* The event loop is no longer shut down while GCD work is in-flight, fixing
  the crash on iOS simulator CI.