Added missing using statement to unit test.

Rx.NET/Source/Tests.System.Reactive/Tests/Linq/ObservableStandardQueryOperatorTest.cs

@@ -10,6 +10,7 @@ using System.Reactive.Linq;
 using System.Reactive.Subjects;
 using System.Text;
 using Microsoft.Reactive.Testing;
+using Microsoft.VisualStudio.TestTools.UnitTesting;
 using ReactiveTests.Dummies;
 
 #if !NO_TPL