Wires up DI for cross wiring correctly ensuring that it occurs at the very end of ConfigureServices, updates tests accordingly, fixes a few other things.

src/Umbraco.Web.UI.NetCore/Startup.cs

@@ -19,8 +19,8 @@ namespace Umbraco.Web.UI.BackOffice
         // For more information on how to configure your application, visit https://go.microsoft.com/fwlink/?LinkID=398940
         public void ConfigureServices(IServiceCollection services)
         {
+            services.AddUmbracoCore();
             services.AddUmbracoWebsite();
-            services.AddUmbracoBackOffice();
         }
 
         // This method gets called by the runtime. Use this method to configure the HTTP request pipeline.