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 think leave ``not_equal`` here for now is fine,  I want to go though other existing tests and see if/how something on ``Subtest`` class could be used.  Maybe we should turn ``failif`` into an callable instance with multiple variations, similar to how unittest has ``assertEqual``, ``assertAlmostEqual``, ``assertTrue``, etc.  Needs more thinking though, maybe just a simple function is good enough.