Clean up tests and fix PR feedback.

src/Markup/Avalonia.Markup.Xaml/MarkupExtensions/BindingExtension.cs

@@ -128,7 +128,6 @@ namespace Avalonia.Markup.Xaml.MarkupExtensions
                 else if (relativeSourceMode == "parent")
                 {
                     relativeSource.Mode = RelativeSourceMode.FindAncestor;
-                    relativeSource.AncestorType = typeof(IControl);
                     relativeSource.AncestorLevel = 1;
                 }
                 else if (relativeSourceMode.StartsWith("parent["))

tests/Avalonia.Markup.Xaml.UnitTests/Xaml/BindingTests_RelativeSource.cs

@@ -152,7 +152,7 @@ namespace Avalonia.Markup.Xaml.UnitTests.Xaml
 
                 // This isn't needed in `Binding_To_First_Ancestor_Without_AncestorType_Uses_LogicalTree` -
                 // why is it needed to get non-null value here?
-                contentControl.ApplyTemplate();
+                //contentControl.ApplyTemplate();
 
                 Assert.Equal("contentControl", button.Content);
             }