Add using

Author: HaoK

src/Localization/Localization/test/ResourceManagerStringLocalizerTest.cs

@@ -8,6 +8,7 @@
 using System.Reflection;
 using System.Resources;
 using Microsoft.AspNetCore.Testing;
+using Microsoft.AspNetCore.Testing.xunit;
 using Microsoft.Extensions.Localization.Internal;
 using Microsoft.Extensions.Logging;
 using Microsoft.Extensions.Logging.Testing;