Avoid race conditions on capture_err reuse

Author: José Valim

lib/elixir/test/elixir/kernel/warning_test.exs

@@ -373,6 +373,16 @@ defmodule Kernel.WarningTest do
     purge Sample
   end
 
+  test "duplicate map keys" do
+    assert capture_err(fn ->
+      defmodule DuplicateKeys do
+        assert %{a: :b, a: :c} == %{a: :c}
+        assert %{1 => 2, 1 => 3} == %{1 => 3}
+        assert %{:a => :b, a: :c} == %{a: :c}
+      end
+    end) =~ "key :a will be overridden in map"
+  end
+
   test "unused guard" do
     assert capture_err(fn ->
       Code.eval_string """

lib/elixir/test/elixir/map_test.exs

@@ -86,16 +86,6 @@ defmodule MapTest do
     assert %{1 => 2, a: :b,} == %{1 => 2, a: :b}
   end
 
-  test "maps with duplicate keys" do
-    ExUnit.CaptureIO.capture_io :stderr, fn ->
-      defmodule DuplicateKeys do
-        assert %{a: :b, a: :c} == %{a: :c}
-        assert %{1 => 2, 1 => 3} == %{1 => 3}
-        assert %{:a => :b, a: :c} == %{a: :c}
-      end
-    end
-  end
-
   test "update maps" do
     assert %{@sample | a: 3} == %{a: 3, b: 2}