Minor change to test code to resolve the xUnit2013 build warning

test/Microsoft.Extensions.Caching.SqlServer.Tests/SqlServerCacheServicesExtensionsTest.cs

@@ -1,10 +1,7 @@
 // Copyright (c) .NET Foundation. All rights reserved.
 // Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
 
-using System;
-using System.IO;
 using System.Linq;
-using System.Threading.Tasks;
 using Microsoft.Extensions.Caching.Distributed;
 using Microsoft.Extensions.DependencyInjection;
 using Moq;
@@ -24,8 +21,7 @@ public void AddDistributedSqlServerCache_AddsAsSingleRegistrationService()
             SqlServerCachingServicesExtensions.AddSqlServerCacheServices(services);
 
             // Assert
-            Assert.Equal(1, services.Count);
-            var serviceDescriptor = services[0];
+            var serviceDescriptor = Assert.Single(services);
             Assert.Equal(typeof(IDistributedCache), serviceDescriptor.ServiceType);
             Assert.Equal(typeof(SqlServerCache), serviceDescriptor.ImplementationType);
             Assert.Equal(ServiceLifetime.Singleton, serviceDescriptor.Lifetime);