Static files: Fix tree to only provide items from expected folders (closes #20962) (#21001)

* Applies checks for root folders to static file tree service.

* Add integration tests.

* Fix ancestor test.

* Amends from code review.

* Integration test compatibility suppressions.

* Reverted breaking change in test base class.

(cherry picked from commit 84c15ff4d7)

tests/Umbraco.Tests.Integration/ManagementApi/Services/Trees/FileSystemTreeServiceTestsBase.cs

@@ -36,21 +36,23 @@ public abstract class FileSystemTreeServiceTestsBase : UmbracoIntegrationTest
             GetStylesheetsFileSystem(),
             GetScriptsFileSystem(),
             null);
+
+        CreateFiles();
+    }
+
+    protected virtual void CreateFiles()
+    {
         for (int i = 0; i < 10; i++)
         {
-            using var stream = CreateStream(Path.Join("tests"));
+            using var stream = CreateStream();
             TestFileSystem.AddFile($"file{i}{FileExtension}", stream);
         }
     }
 
     protected static Stream CreateStream(string contents = null)
     {
-        if (string.IsNullOrEmpty(contents))
-        {
-            contents = "/* test */";
-        }
-
-        var bytes = Encoding.UTF8.GetBytes(contents);
+        const string DefaultFileContent = "/* test */";
+        var bytes = Encoding.UTF8.GetBytes(contents ?? DefaultFileContent);
         return new MemoryStream(bytes);
     }