Add a unit test to prove the fix is essential.

Author: Lifeng Lu

test/Microsoft.VisualStudio.Threading.Tests/JoinableTaskCollectionTests.cs

@@ -196,4 +196,21 @@ public void JoinDisposedTwice()
             return Task.CompletedTask;
         });
     }
+
+    [Fact]
+    public void JoinTillEmptyWorksWithRefCounting()
+    {
+        var finishTaskEvent = new AsyncManualResetEvent();
+        JoinableTask? task = this.JoinableFactory.RunAsync(async delegate { await finishTaskEvent.WaitAsync().ConfigureAwait(false); });
+
+        var collection = new JoinableTaskCollection(this.context, refCountAddedJobs: true);
+
+        collection.Add(task);
+        collection.Add(task);
+
+        finishTaskEvent.Set();
+
+        Task? waiter = collection!.JoinTillEmptyAsync();
+        Assert.True(waiter.Wait(UnexpectedTimeout));
+    }
 }