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'm not executing echo, I'm just putting few strings (send vs. sendline) into first session. 

Sorry, should have been more explicit.  I was thinking more of in-line substitutions inside ``cmd()``'s command. e.g. ``tabtabtab=$(echo -ne '\t\t\t')``.  Though yeah, even that's probably too specific to be of much use.  I was mostly just  feeling sorry for that poor lonely ``InteractiveBash`` class, all sad with it's one little method :cry:  hehehe.