Cleanup to skip/flaky attributes (dotnet/extensions#2186)

\n\nCommit migrated from dotnet/extensions@cfef5e0

Author: jkotalik

src/Caching/SqlServer/test/SqlServerCacheWithDatabaseTest.cs

@@ -6,7 +6,7 @@
 using System.Data;
 using System.Text;
 using System.Threading.Tasks;
-using Microsoft.AspNetCore.Testing.xunit;
+using Microsoft.AspNetCore.Testing;
 using Microsoft.Data.SqlClient;
 using Microsoft.Extensions.Caching.Distributed;
 using Microsoft.Extensions.Configuration;