Creates new Umbraco.Tests.Common netstandard project to host the common testing functionality between netframework and netcore, this will be ongoing work (doesn't need to be done all at once)

src/Umbraco.Tests/Web/WebExtensionMethodTests.cs

@@ -7,6 +7,7 @@ using System.Web.Routing;
 using Moq;
 using NUnit.Framework;
 using Umbraco.Core.Services;
+using Umbraco.Tests.Common;
 using Umbraco.Tests.TestHelpers;
 using Umbraco.Tests.TestHelpers.Stubs;
 using Umbraco.Tests.Testing;