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>
I don't see it that way at all, there are plenty of other methods here that go a long way to help support async. interaction.  Similar to ``is_responsive()`` I just think it's trying to do too much work.  What about defining it as an abstract method, raising ``NotImplementedError`` then let sub-classes define it if it's relevant to their purpose (e.g. I  see it would have use inside ``InteractiveBash``).