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, well for this test base-class alone, you're well under 200 enough that it could be split out for easier readability.  I think you'll find it'll let you cut way back on the code needed in ``basic`` and ``stress`` as well since all the similar bits can be re-used.  Either way, IMHO you met the "simpler" criteria well enough, I'm quite happy.