Fixed #33719 -- Fixed test command crash when running in parallel.

Thanks Pēteris Caune for the report.

Regression in 3b3f38b3b0.

tests/test_runner/tests.py

@@ -19,6 +19,7 @@ from django.test import SimpleTestCase, TransactionTestCase, skipUnlessDBFeature
 from django.test.runner import (
     DiscoverRunner,
     Shuffler,
+    _init_worker,
     reorder_test_bin,
     reorder_tests,
     shuffle_tests,
@@ -684,6 +685,46 @@ class NoInitializeSuiteTestRunnerTests(SimpleTestCase):
             )
 
 
+class TestRunnerInitializerTests(SimpleTestCase):
+
+    # Raise an exception to don't actually run tests.
+    @mock.patch.object(
+        multiprocessing, "Pool", side_effect=Exception("multiprocessing.Pool()")
+    )
+    def test_no_initialize_suite_test_runner(self, mocked_pool):
+        class StubTestRunner(DiscoverRunner):
+            def setup_test_environment(self, **kwargs):
+                return
+
+            def setup_databases(self, **kwargs):
+                return
+
+            def run_checks(self, databases):
+                return
+
+            def teardown_databases(self, old_config, **kwargs):
+                return
+
+            def teardown_test_environment(self, **kwargs):
+                return
+
+            def run_suite(self, suite, **kwargs):
+                kwargs = self.get_test_runner_kwargs()
+                runner = self.test_runner(**kwargs)
+                return runner.run(suite)
+
+        runner = StubTestRunner(verbosity=0, interactive=False, parallel=2)
+        with self.assertRaisesMessage(Exception, "multiprocessing.Pool()"):
+            runner.run_tests(
+                [
+                    "test_runner_apps.sample.tests_sample.TestDjangoTestCase",
+                    "test_runner_apps.simple.tests",
+                ]
+            )
+        # Initializer must be a function.
+        self.assertIs(mocked_pool.call_args.kwargs["initializer"], _init_worker)
+
+
 class Ticket17477RegressionTests(AdminScriptTestCase):
     def setUp(self):
         super().setUp()