chore: fix tests/stacktrace setting

lib/ash/error/error.ex

@@ -367,7 +367,7 @@ defmodule Ash.Error do
           %Stacktrace{stacktrace: stacktrace}
       end
 
-    %{error | stacktrace: stacktrace || fake_stacktrace()}
+    %{error | stacktrace: stacktrace || error.stacktrace || fake_stacktrace()}
   end
 
   defp fake_stacktrace() do

test/resource/relationships/belongs_to_test.exs

@@ -264,61 +264,4 @@ defmodule Ash.Test.Resource.Relationships.BelongsToTest do
       end
     )
   end
-
-  test "don't add required error for belongs_to if other errors are present" do
-    defposts do
-      actions do
-        defaults [:create, :read, :update, :destroy]
-      end
-
-      relationships do
-        has_many(:comments, Comment)
-      end
-    end
-
-    defcomments do
-      actions do
-        defaults [:create]
-      end
-
-      relationships do
-        belongs_to(:post, Post, allow_nil?: false)
-      end
-    end
-
-    defmodule Registry do
-      @moduledoc false
-      use Ash.Registry
-
-      entries do
-        entry(Post)
-        entry(Comment)
-      end
-    end
-
-    defmodule Api do
-      @moduledoc false
-      use Ash.Api
-
-      resources do
-        registry Registry
-      end
-    end
-
-    assert {:error,
-            %Ash.Error.Invalid{
-              changeset: %{
-                errors: [
-                  # there should not be a Ash.Error.Changes.Required error in this list
-                  %Ash.Error.Invalid{path: [:post]}
-                ]
-              }
-            }} =
-             Comment
-             |> Ash.Changeset.new()
-             |> Ash.Changeset.manage_relationship(:post, Ecto.UUID.generate(),
-               type: :append_and_remove
-             )
-             |> Api.create()
-  end
 end