[3.9] GH-89074: Fixed IsolatedAsyncioTestCase from throwing an exception on leaked tasks (GH-27765) (#91471)

(cherry picked from commit 2cb1a6806c)

Co-authored-by: Bar Harel <bar.harel@biocatch.com>

Lib/unittest/test/test_async_case.py

@@ -278,6 +278,26 @@ class TestAsyncCase(unittest.TestCase):
         output = test.run()
         self.assertFalse(output.wasSuccessful())
 
+    def test_cancellation_hanging_tasks(self):
+        cancelled = False
+        class Test(unittest.IsolatedAsyncioTestCase):
+            async def test_leaking_task(self):
+                async def coro():
+                    nonlocal cancelled
+                    try:
+                        await asyncio.sleep(1)
+                    except asyncio.CancelledError:
+                        cancelled = True
+                        raise
+
+                # Leave this running in the background
+                asyncio.create_task(coro())
+
+        test = Test("test_leaking_task")
+        output = test.run()
+        self.assertTrue(cancelled)
+
+
 
 if __name__ == "__main__":
     unittest.main()