Fix testSchedulingTaskOnSleepingLoopWakesUpOnce (#1992)

Motivation:

This test is flaky, and has always been flaky.

The issue here is simply a timing one. It has always been possible
for the thread running in the background DispatchQueue to be take
longer between calling semaphore.signal and promise.cascade(to:)
than it does for the event loop to process the tasks. If that
happens, this test will fail by eventually timing out.

Modifications:

- Adjust the test to avoid using promise.cascade(to:), and so prevent it
  from having at timing window.

Result:

The test will be not flaky, or at least less flaky.

Resolves #1971.

Co-authored-by: George Barnett <gbarnett@apple.com>

Tests/NIOPosixTests/SALEventLoopTests.swift

@@ -66,12 +66,13 @@ final class SALEventLoopTests: XCTestCase, SALTest {
                 }
 
                 // Now enqueue a "last" task.
-                let lastTask = thisLoop.submit { i &+= 1 }
+                thisLoop.execute {
+                    i &+= 1
+                    promise.succeed(())
+                }
 
                 // Now we can unblock the semaphore.
                 semaphore.signal()
-
-                lastTask.cascade(to: promise)
             }
             
             return promise.futureResult