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 prefer this way as you can simply follow it and see what is going on. Anyway I' fine with splitting it into separate functions. Again, this test is to show you the bare minimum required for an interactive test to work. `dexpect` version is the real version. (for interaction test you need all of these functions except the `InteractiveBash` class).