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>
When you set timeout=0, exactly one read is executed. So the behavior is exactly the same, only it logs the output. Initial version didn't have `_try_read` at all and it was directly in the loop. When I simplified the loop there was need to use this code twice so I created the function.

Again, it's always easier to add public function, than get rid of one.