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>
This class uses the `subtest`'s logging. It only adds the prefix and print's using `%r` to print non-printable chars. It's way easier to fix it on one place, than trying to make it work in all (just) 3 places. Additionally try to fit this function in ~40 chars long line. Noone would be able to read it.

See this function in `dexpect` it supports multiple `_log` functions multiple variants including message storing or quite.