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>
You mean to have an `exit` function, which look like:
```python
def exit(self):
    self.sendline('exit')
```
That seems as wasting and people might get confused whether you force-close the session, or just check for proper cleanup. The check for end is required only when you finish the last bash/function you executed. And unlike `cmd` it fits lambda + wait_for pretty easily.