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>
"Should" != "Must", maybe there is a reason to override or extend at the ``_try_read()`` point, we can never predict.  I prefer to make everything public unless there's some really good reason not too (like things it must always be called in a specific order or something breaks.)