asyncio: Close transports in tests

* Use test_utils.run_briefly() to execute pending calls to really close
  transports
* sslproto: mock also _SSLPipe.shutdown(), it's need to close the transport
* pipe test: the test doesn't close explicitly the PipeHandle, so ignore
  the warning instead
* test_popen: use the context manager ("with p:") to explicitly close pipes

Lib/test/test_asyncio/test_subprocess.py

@@ -286,6 +286,7 @@ class SubprocessMixin:
         # "Exception during subprocess creation, kill the subprocess"
         with test_utils.disable_logger():
             self.loop.run_until_complete(cancel_make_transport())
+            test_utils.run_briefly(self.loop)
 
 
 if sys.platform != 'win32':