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 don't care a toad's mouldy foot about compatibility with virttest or Bash'isms.  Our re-use domain is only within the same subtest, within dockertest, then within autotest (in that order).  I'd much rather give the control/choice on how and when to loop/timeout to the test.  Though perhaps we could compromise by making ``_try_read()`` public, then at least there's an available circumvention (of the outside timeout) when it's not needed.  Could that work?