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>
That's why `dexpect` has cmd, cmd_output, cmd_status and cmd_status_output functions. This is for simplification. In bash when you execute a function/program, output and exit_code are the most basic interface you have and need.