Testable interface should have a testableCloseSocket() method

The ClientCnxn class has a protected function testableCloseSocket(), which allows applications to test how their system will respond if the Zookeeper socket connection is closed.

This would be a very easy feature to add to the Testable interface, as the ZookeeperTestable class has access to a ClientCnxn that it could easily pass the testableCloseSocket() call to. That way applications wanting to test this do not need to rely on the tests artifacts in order to use this method, as they currently do.