Fix order in ExUnit results when listing pinned variables (#14723)

The pinned variables were returned in a random order (often reversed):

     test/ex_unit_pinned_variables_order_test.exs:23
     match (=) failed
     The following variables were pinned:
       var_d = "four"
       var_c = "three"
       var_b = "two"
       var_a = "one"
     code:  assert %{a: ^var_d, b: ^var_c, c: ^var_b, d: ^var_a} = build(var_a, var_b, var_c, var_d)
     left:  %{a: ^var_d, b: ^var_c, c: ^var_b, d: ^var_a}
     right: %{a: "one", b: "two", c: "three", d: "four"}
     stacktrace:
       test/ex_unit_pinned_variables_order_test.exs:29: (test)

This fix sorts them alphabetically.

This bug was introduced in 884e933 when the pinned vars were now accumulated in a map (instead
of a list).

A repo replicating the issue can be found here:
- https://github.com/eksperimental-debug/elixir_debug/tree/ex-unit-pinned-variables-order
- https://github.com/eksperimental-debug/elixir_debug/blob/ex-unit-pinned-variables-order/ex_unit_pinned_variables_order/test/ex_unit_pinned_variables_order_test.exs

Author: eksperimental

lib/ex_unit/lib/ex_unit/assertions.ex

@@ -624,7 +624,7 @@ defmodule ExUnit.Assertions do
   def __pins__(pins) do
     pins
     |> Enum.filter(fn {{_, ctx}, _} -> ctx == nil end)
-    |> Enum.reverse()
+    |> Enum.sort()
     |> Enum.map_join(@indent, fn {{name, _}, var} -> "#{name} = #{inspect(var)}" end)
     |> case do
       "" ->

lib/ex_unit/test/ex_unit/assertions_test.exs

@@ -524,8 +524,8 @@ defmodule ExUnit.AssertionsTest do
         """
         Assertion failed, no matching message after 0ms
         The following variables were pinned:
-          status = :valid
           other_status = :invalid
+          status = :valid
         Showing 1 of 1 message in the mailbox\
         """ = error.message