Avoid using Keyword.pop/2 in ExUnit.Diff.struct_module/2 (#15489)

`struct_module` is being called for both structs and maps.
`diff_quoted_struct` uses the return of `struct_module` to determine
when to call `diff_quoted_struct` or `diff_map`.

Author: tudborg

lib/ex_unit/lib/ex_unit/diff.ex

@@ -765,16 +765,20 @@ defmodule ExUnit.Diff do
     end
   end
 
+  # if kw represents a struct, this returns the result of `struct.__info__(:struct)`, otherwise nil
   defp struct_module(kw) do
-    {struct, struct_kw} = Keyword.pop(kw, :__struct__)
-
-    info =
-      is_atom(struct) and struct != nil and
-        Code.ensure_loaded?(struct) and function_exported?(struct, :__info__, 1) and
-        struct.__info__(:struct)
+    case Enum.split_with(kw, fn {k, _} -> k == :__struct__ end) do
+      {[{_, struct} | _], struct_kw} when is_atom(struct) and struct != nil ->
+        info =
+          Code.ensure_loaded?(struct) and function_exported?(struct, :__info__, 1) and
+            struct.__info__(:struct)
+
+        if info && Enum.all?(struct_kw, fn {k, _} -> Enum.any?(info, &(&1.field == k)) end) do
+          struct
+        end
 
-    if info && Enum.all?(struct_kw, fn {k, _} -> Enum.any?(info, &(&1.field == k)) end) do
-      struct
+      _ ->
+        nil
     end
   end