asyncio/tests: Ensure a gc_collect for __del__ testing

Patch by Philip Jenvey

Lib/test/test_asyncio/test_base_events.py

@@ -628,6 +628,7 @@ class BaseEventLoopTests(test_utils.TestCase):
             fut.add_done_callback(lambda *args: self.loop.stop())
             self.loop.run_forever()
             fut = None # Trigger Future.__del__ or futures._TracebackLogger
+            support.gc_collect()
             if PY34:
                 # Future.__del__ in Python 3.4 logs error with
                 # an actual exception context

Lib/test/test_asyncio/test_futures.py

@@ -242,6 +242,7 @@ class FutureTests(test_utils.TestCase):
         fut.set_exception(RuntimeError('boom'))
         del fut
         test_utils.run_briefly(self.loop)
+        support.gc_collect()
         self.assertTrue(m_log.error.called)
 
     @mock.patch('asyncio.base_events.logger')