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>
I probably miss understood the `wait_for_output`. How is it different from `read_nonblocking`? I mean `read_nonblocking` is the key element of the stream control and the difference from only wait_for_it or actually do it is minimal.