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>
Yes, that would be quite pointless :)   I was originally referring to the larger section, starting around``dkrcmd.is_responsive()``.  Reading it again, it's too specific to this test, so nevermind, sorry I didn't look closely enough.