[6.0.x] Fixed #36491 -- Fixed crash in ParallelTestRunner with --buffer.

Thanks Javier Buzzi and Adam Johnson for reviews.

Co-authored-by: Simon Charette <charette.s@gmail.com>

Backport of be581ff473 from main.

tests/test_runner/test_parallel.py

@@ -282,3 +282,27 @@ class ParallelTestSuiteTest(SimpleTestCase):
 
         self.assertEqual(len(result.errors), 0)
         self.assertEqual(len(result.failures), 0)
+
+    def test_buffer_mode_reports_setupclass_failure(self):
+        test = SampleErrorTest("dummy_test")
+        remote_result = RemoteTestResult()
+        suite = TestSuite([test])
+        suite.run(remote_result)
+
+        pts = ParallelTestSuite([suite], processes=2, buffer=True)
+        pts.serialized_aliases = set()
+        test_result = TestResult()
+        test_result.buffer = True
+
+        with unittest.mock.patch("multiprocessing.Pool") as mock_pool:
+
+            def fake_next(*args, **kwargs):
+                test_result.shouldStop = True
+                return (0, remote_result.events)
+
+            mock_pool.return_value.imap_unordered.return_value = unittest.mock.Mock(
+                next=fake_next
+            )
+            pts.run(test_result)
+
+        self.assertIn("ValueError: woops", test_result.errors[0][1])