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 an aweful lot of work for a base-class to perform.  Suggest breaking this up into 3/4 methods would make re-using this class easier.   Otherwise it's "all-or-nothing" if a subclass wants to call ``super().initialize()`` or it just ends up overriding  the entire thing.