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>
Lets call it people __must__ always use `read_nonblocking` which with certain setting does the same job as `_try_read`. When in the future there is desire for it, we can make it public. It's always easier to add public function, than get rid of one.