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>
In `tty` mode this function was pretty simple. But look at this function and try to narrow it down to simple `wait_for` function. It took me pretty long time to develop this one (which is still not perfect, ... see the pure `Exception`, it should probably disappear or at least say, send the log to ldoktor when you see this).

I'm not in favor of useless methods, but in this case it's really useful. For example when you start using session, when container is not yet initialized, it just flushes the stdin after init and you end up with partial writes... So you should actually call this function whenever you want to use session for the first time. The same with executing another bash inside the container. It flushes the stdin, so it's safer to execute this function and see, when it's really initialized.