bpo-37137: Fix test_asyncio: use TestCase.set_event_loop() (GH-13779)

Replace asyncio.set_event_loop() with TestCase.set_event_loop() of
test_asyncio.utils: this method calls TestCase.close_loop() which
waits until the executor completes, to avoid leaking dangling
threads.

Inherit from test_asyncio.utils.TestCase rather than
unittest.TestCase.

Lib/test/test_asyncio/test_tasks.py

@@ -2775,7 +2775,7 @@ def test__unregister_task_not_registered(self):
         self.assertEqual(asyncio.all_tasks(loop), set())
 
 
-class PyIntrospectionTests(unittest.TestCase, BaseTaskIntrospectionTests):
+class PyIntrospectionTests(test_utils.TestCase, BaseTaskIntrospectionTests):
     _register_task = staticmethod(tasks._py_register_task)
     _unregister_task = staticmethod(tasks._py_unregister_task)
     _enter_task = staticmethod(tasks._py_enter_task)
@@ -2784,7 +2784,7 @@ class PyIntrospectionTests(unittest.TestCase, BaseTaskIntrospectionTests):
 
 @unittest.skipUnless(hasattr(tasks, '_c_register_task'),
                      'requires the C _asyncio module')
-class CIntrospectionTests(unittest.TestCase, BaseTaskIntrospectionTests):
+class CIntrospectionTests(test_utils.TestCase, BaseTaskIntrospectionTests):
     if hasattr(tasks, '_c_register_task'):
         _register_task = staticmethod(tasks._c_register_task)
         _unregister_task = staticmethod(tasks._c_unregister_task)
@@ -2799,12 +2799,7 @@ class BaseCurrentLoopTests:
     def setUp(self):
         super().setUp()
         self.loop = asyncio.new_event_loop()
-        asyncio.set_event_loop(self.loop)
-
-    def tearDown(self):
-        self.loop.close()
-        asyncio.set_event_loop(None)
-        super().tearDown()
+        self.set_event_loop(self.loop)
 
     def new_task(self, coro):
         raise NotImplementedError
@@ -2828,15 +2823,15 @@ async def coro():
         self.assertIsNone(asyncio.current_task(loop=self.loop))
 
 
-class PyCurrentLoopTests(BaseCurrentLoopTests, unittest.TestCase):
+class PyCurrentLoopTests(BaseCurrentLoopTests, test_utils.TestCase):
 
     def new_task(self, coro):
         return tasks._PyTask(coro, loop=self.loop)
 
 
 @unittest.skipUnless(hasattr(tasks, '_CTask'),
                      'requires the C _asyncio module')
-class CCurrentLoopTests(BaseCurrentLoopTests, unittest.TestCase):
+class CCurrentLoopTests(BaseCurrentLoopTests, test_utils.TestCase):
 
     def new_task(self, coro):
         return getattr(tasks, '_CTask')(coro, loop=self.loop)
@@ -3245,7 +3240,7 @@ class SleepTests(test_utils.TestCase):
     def setUp(self):
         super().setUp()
         self.loop = asyncio.new_event_loop()
-        asyncio.set_event_loop(None)
+        self.set_event_loop(self.loop)
 
     def tearDown(self):
         self.loop.close()
@@ -3279,7 +3274,7 @@ class WaitTests(test_utils.TestCase):
     def setUp(self):
         super().setUp()
         self.loop = asyncio.new_event_loop()
-        asyncio.set_event_loop(None)
+        self.set_event_loop(self.loop)
 
     def tearDown(self):
         self.loop.close()
@@ -3306,7 +3301,7 @@ class CompatibilityTests(test_utils.TestCase):
     def setUp(self):
         super().setUp()
         self.loop = asyncio.new_event_loop()
-        asyncio.set_event_loop(None)
+        self.set_event_loop(self.loop)
 
     def tearDown(self):
         self.loop.close()