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>
Here you go, this test could easily re-use/share a method in base-class with ``basic``.  Heck, it's even bash-centric enough you could get away with putting it right onto that poor, lonely ``InteractiveBash`` class :smile: