Override the connect_read_pipe() method of the loop to mock immediatly
pause_reading() and resume_reading() methods.

The test failed randomly on FreeBSD 9 buildbot and on Windows using trollius.

* 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

cancelled waiter.

Add unit test cancelling subprocess methods.

future before setting its result.

In Tulip, the code is different because it is exactly the same code in CPython,
and CPython doesn't provide asyncio.test_support.

Don't use directly test.support because Python 3.2 doesn't provide IPV6_ENABLED
for example.