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 see "difficulty to write", and "multiple exception handling" as a clue it's trying to solve a large a problem at too low of a level.  I'd like to see this moved to the higher-level like ``InteractiveBash()``.  Remember too that "features" almost always  creep "up" but rarely "down".  If we really do need this method here, it's easy to put back.  Finally, for all practical purposes, testing will determine if something is non-responsive because another operation will ultimately fail.  That will help detect we have timing dependency in the test, which is something we need to know about, not hide.