[Concurrency] Fix cooperative executor to return only after all jobs run.

We were terminating after the first set of jobs; if one of them scheduled
another job, and there were no timers running, we would terminate,
which was wrong.

Author: al45tair

stdlib/public/Concurrency/CooperativeExecutor.swift

@@ -265,13 +265,15 @@ extension CooperativeExecutor: RunLoopExecutor {
           _sleep(seconds: toWait.seconds,
                  nanoseconds: toWait.nanoseconds)
         }
-      } else {
+      } else if runQueue.isEmpty {
         // Stop if no more jobs are available
         break
       }
       #else // $Embedded || SWIFT_STDLIB_TASK_TO_THREAD_MODEL_CONCURRENCY
-      // Stop if no more jobs are available
-      break
+      if runQueue.isEmpty {
+        // Stop if no more jobs are available
+        break
+      }
       #endif
     }
   }

stdlib/public/Concurrency/PriorityQueue.swift

@@ -60,6 +60,11 @@ struct PriorityQueue<T> {
     upHeap(ndx: storage.count - 1)
   }
 
+  /// Return `true` if the queue is empty
+  var isEmpty: Bool {
+    return storage.isEmpty
+  }
+
   /// The highest priority item from the queue, or `nil` if none.
   var top: T? {
     if storage.isEmpty {