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>
For me `basic` means, it's the simplest generic version of this test and you should run at least this one to see, whether this feature works. Something like smoke test. The rest is specific and focused on some part or specific bug of this feature.