NetCore: MSDI refactor split CoreRuntime & Remove Current (#9356)

* Split Bootstrap code from CoreRuntime

* Make ContentService method signatures match interface
prevents need to hide the concrete class in tests

* Remove Current

Only bit that bugs me is TreeNode for IconFilePath
I get the feeling it's dead code, but can just use a setter in cs or work out path in js if required.

* Fix tests, remember to terminate IRuntime

src/Umbraco.Tests.Integration/TestServerTest/UmbracoTestServerTestBase.cs

@@ -13,7 +13,6 @@ using Microsoft.AspNetCore.TestHost;
 using Microsoft.Extensions.DependencyInjection;
 using Microsoft.Extensions.DependencyInjection.Extensions;
 using NUnit.Framework;
-using Umbraco.Composing;
 using Umbraco.Core;
 using Umbraco.Extensions;
 using Umbraco.Tests.Integration.Testing;
@@ -118,13 +117,9 @@ namespace Umbraco.Tests.Integration.TestServerTest
         public override void TearDown()
         {
             base.TearDown();
+            base.TerminateCoreRuntime();
 
             Factory.Dispose();
-
-            if (Current.IsInitialized)
-            {
-                Current.IsInitialized = false;
-            }
         }
 
         #region IStartup