Add additional test cases (#125)

Test-only changes for code coverage.  Please review the comment and I'll include the changes in the next iteration.

* Add more tests

* Add some more additional test cases

test/Microsoft.SqlTools.ServiceLayer.Test/ServiceHost/AsyncLockTests.cs

@@ -0,0 +1,49 @@
+//
+// Copyright (c) Microsoft. All rights reserved.
+// Licensed under the MIT license. See LICENSE file in the project root for full license information.
+//
+
+using System;
+using System.Threading;
+using System.Threading.Tasks;
+using Microsoft.SqlTools.ServiceLayer.Utility;
+using Xunit;
+
+namespace Microsoft.SqlTools.ServiceLayer.Test.ServiceHost
+{
+    public class AsyncLockTests
+    {
+        [Fact]
+        public async Task AsyncLockSynchronizesAccess()
+        {
+            AsyncLock asyncLock = new AsyncLock();
+
+            Task<IDisposable> lockOne = asyncLock.LockAsync();
+            Task<IDisposable> lockTwo = asyncLock.LockAsync();
+
+            Assert.Equal(TaskStatus.RanToCompletion, lockOne.Status);
+            Assert.Equal(TaskStatus.WaitingForActivation, lockTwo.Status);
+            lockOne.Result.Dispose();
+
+            await lockTwo;
+            Assert.Equal(TaskStatus.RanToCompletion, lockTwo.Status);
+        }
+
+        [Fact]
+        public void AsyncLockCancelsWhenRequested()
+        {
+            CancellationTokenSource cts = new CancellationTokenSource();
+            AsyncLock asyncLock = new AsyncLock();
+
+            Task<IDisposable> lockOne = asyncLock.LockAsync();
+            Task<IDisposable> lockTwo = asyncLock.LockAsync(cts.Token);
+
+            // Cancel the second lock before the first is released
+            cts.Cancel();
+            lockOne.Result.Dispose();
+
+            Assert.Equal(TaskStatus.RanToCompletion, lockOne.Status);
+            Assert.Equal(TaskStatus.Canceled, lockTwo.Status);
+        }
+    }
+}