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>
Here as well, it seems useful on the surface, but it's really trying to anticipate needs too much.  IMHO, it's better for these kinds of classes to do as little "work" as possible.  Leave it to sub-classes to extend further, or just leave it as an opening for the caller to connect things together.