subtests.docker_cli.detach: New detach test

This patch adds basic detach test. There are 3 variants:
* basic - basic ctrl+p ctrl+q test
* stress - many iterations (currently only 3 as ctrl+p ctrl+q requires
           sleep)
* multi_session - uses 2 session attached to single container
                  simultaneously

Signed-off-by: Lukáš Doktor <ldoktor@redhat.com>
Not sure this method is needed, couldn't someone just wire up ``output.wait_for_output(instance.stdout()...)```?  Just thinking from the "do as little work as possible" perspective.  It also lets us re-use ``wait_for_output()`` method more instead of doing the same thing slightly differently.