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>
Well without the name you'd be lost in case of multiple consoles or multiple dockers. And about the developers, I don't know how about libvirt ones (they are probably close to python) but qemu ones never touch or read the autotest test. So the description has to be neat and well understood without the need to look into the code. (I guess that docker-go guys don't like python either)