CSHARP-5699: Add System.Linq by default into LinqIntegrationTest template

Author: Oleksandr Poliakov

CSharpDriver.sln.DotSettings

@@ -120,6 +120,7 @@ namespace $NAMESPACE$
 */
 
 using System.Collections.Generic;
+using System.Linq;
 using MongoDB.Driver.TestHelpers;
 using FluentAssertions;
 using Xunit;