Improve testTasksScheduledDuringShutdownAreAutomaticallyCancelled

Motivation:

NIOAsyncTestingEventLoopTests.testTasksScheduledDuringShutdownAreAutomaticallyCancelled
is flaky.

The recursivesly schedules tasks to run on the event loop and then,
after a small pause, shuts down the event loop. It then asserts that
more then 1 task was scheduled (i.e. at least 1 recursive task was
run). This assertion occasionally fails as exactly 1 task was run.

I haven't been able to reproduce this locally but I believe the root of
the flakiness is that the child tasks to shutdown the loop and advance
the time (to trigger the recursive scheduling) can race. If the shutdown
wins then only the root task will run.

Modifications:

- Remove the array of scheduled tasks, it wasn't being used
- Fix atomic ordering
- Increase sleep time before shutting down
- Advance time in the task group rather than a child task

Result:

Less flaky test (hopefully)

Tests/NIOEmbeddedTests/AsyncTestingEventLoopTests.swift

@@ -481,19 +481,16 @@ final class NIOAsyncTestingEventLoopTests: XCTestCase {
     func testTasksScheduledDuringShutdownAreAutomaticallyCancelled() async throws {
         let eventLoop = NIOAsyncTestingEventLoop()
         let tasksRun = ManagedAtomic(0)
-        var childTasks: [Scheduled<Void>] = []
 
         func scheduleRecursiveTask(
             at taskStartTime: NIODeadline,
             andChildTaskAfter childTaskStartDelay: TimeAmount
         ) -> Scheduled<Void> {
             eventLoop.scheduleTask(deadline: taskStartTime) {
-                tasksRun.wrappingIncrement(ordering: .relaxed)
-                childTasks.append(
-                    scheduleRecursiveTask(
-                        at: eventLoop.now + childTaskStartDelay,
-                        andChildTaskAfter: childTaskStartDelay
-                    )
+                tasksRun.wrappingIncrement(ordering: .releasing)
+                _ = scheduleRecursiveTask(
+                    at: eventLoop.now + childTaskStartDelay,
+                    andChildTaskAfter: childTaskStartDelay
                 )
             }
         }
@@ -502,17 +499,15 @@ final class NIOAsyncTestingEventLoopTests: XCTestCase {
 
         try await withThrowingTaskGroup(of: Void.self) { group in
             group.addTask {
-                try await Task.sleep(nanoseconds: 1_000_000)
+                try await Task.sleep(nanoseconds: 10_000_000)
                 await eventLoop.shutdownGracefully()
             }
-            group.addTask {
-                await eventLoop.advanceTime(to: .uptimeNanoseconds(1))
-            }
+
+            await eventLoop.advanceTime(to: .uptimeNanoseconds(1))
             try await group.waitForAll()
         }
 
-        XCTAssertGreaterThan(tasksRun.load(ordering: .relaxed), 1)
-        XCTAssertEqual(childTasks.count, tasksRun.load(ordering: .relaxed))
+        XCTAssertGreaterThan(tasksRun.load(ordering: .acquiring), 1)
     }
 
     func testShutdownCancelsRemainingScheduledTasks() async {