bpo-40115: Fix refleak in test_asyncio.test_run_in_executor_cancel() (GH-19282)

Call explicitly self.loop.shutdown_default_executor().

Author: aeros

Lib/test/test_asyncio/test_events.py

@@ -365,6 +365,8 @@ def run():
 
         f2 = self.loop.run_in_executor(None, run)
         f2.cancel()
+        self.loop.run_until_complete(
+                self.loop.shutdown_default_executor())
         self.loop.close()
         self.loop.call_soon = patched_call_soon
         self.loop.call_soon_threadsafe = patched_call_soon