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>
This is a good idea for a method on ``SubBase`` class.  Only suggestion is change ``self`` -> ``subtest`` so there's no confusion, and we can drop this right into the class later.