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>
Ahh okay, here you go.  This class would be a MUCH better place to stick that ``cmd()`` and ``is_responsive()`` methods.  They are much closer tied to bash-specific stuff then 'generic interactive thing'.  It also makes this class more important, it was feeling very lonely having just one property :cry: