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>
OK, I found the function in latest master. It does very similar job, but it's not cumulative. And as the regexp can be multiline you might never finish. Also you won't get the full output after the `wait_for_output`. I like reusing code, but currently there is only one user of cumulative match...