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>
These kinds of methods initially seem to be very useful, but in my experience they're much less so.  It's easy for tests to sub-class and add such things if they need them.  Otherwise, we already have several "wait for something" utilities scattered about, so it's probably fine to just let the caller figure out how best to do it.