Wait until test process is down in ExUnit runner

This should fix a race condition where the next test would
start before all linked processes in the previous test were
killed, often leading to already registered errors.

Author: José Valim

lib/ex_unit/lib/ex_unit/runner.ex

@@ -159,7 +159,9 @@ defmodule ExUnit.Runner do
     {test_case, pending} =
       receive do
         {^case_pid, :case_finished, test_case, tests} ->
-          Process.demonitor(case_ref, [:flush])
+          receive do
+            {:DOWN, ^case_ref, :process, ^case_pid, _} -> :ok
+          end
           {test_case, tests}
         {:DOWN, ^case_ref, :process, ^case_pid, error} ->
           test_case = %{test_case | state: {:failed, {{:EXIT, case_pid}, error, []}}}
@@ -214,7 +216,9 @@ defmodule ExUnit.Runner do
     test =
       receive do
         {^test_pid, :test_finished, test} ->
-          Process.demonitor(test_ref, [:flush])
+          receive do
+            {:DOWN, ^test_ref, :process, ^test_pid, _} -> :ok
+          end
           test
         {:DOWN, ^test_ref, :process, ^test_pid, error} ->
           %{test | state: {:failed, {{:EXIT, test_pid}, error, []}}}