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>
The only thing the docker engineers will see is a test name and PASS/FAIL.  Everything on the console and in the debug log is more-or-less only ever going to be seen by folks writing and running the tests.  In any case it's just a small thing, having the docstrings correct is much more important, since those are certainly intended for test-writers.