Fix regression when diffing nested improper lists (#14292)

Close #14291

Author: sabiwara

lib/ex_unit/lib/ex_unit/diff.ex

@@ -1162,6 +1162,7 @@ defmodule ExUnit.Diff do
     else
       other
       |> Map.to_list()
+      |> Enum.map(&escape_pair/1)
       |> build_map_or_struct(struct)
     end
   end

lib/ex_unit/test/ex_unit/diff_test.exs

@@ -335,6 +335,8 @@ defmodule ExUnit.DiffTest do
       "[[[[], \"Hello-,- \"] | \"world\"] | \"!\"]",
       "[[[[], \"Hello \"] | \"world\"] | \"!\"]"
     )
+
+    refute_diff(:foo = %{bar: [:a | :b]}, "", "")
   end
 
   test "proper lists" do

lib/ex_unit/test/ex_unit/formatter_test.exs

@@ -510,6 +510,17 @@ defmodule ExUnit.FormatterTest do
            """
   end
 
+  test "formats assertions with nested improper list diffing" do
+    failure = [{:error, catch_assertion(assert :foo = %{bar: [1 | 2]}), []}]
+
+    assert format_test_all_failure(test_module(), failure, 1, :infinity, &diff_formatter/2) =~ """
+           match (=) failed
+                code:  assert :foo = %{bar: [1 | 2]}
+                left:  :foo
+                right: %{bar: [1 | 2]}
+           """
+  end
+
   defmodule BadInspect do
     defstruct key: 0