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>
In addition to testing the same thing multiple times, it's also useful for performing different actions.  For example if "iteration" number is an index into a list of commands to run.  Taken further, it doesn't need to be an integer range either, it could be any iterable.  So you could iterate over keys in a dictionary that are the commands to run, and the values are the expected results (or some such).  Anyway, it was just an idea, and they're cheap :)