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 know about you, but I use the bash ``echo`` command all-the-frickin'-time.  Maybe this would be good as a method on ``InteractiveBash`` class?  Or not.  Just a thought.