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 has nothing to do with `InteractiveBash`. You need to call this function before you start using `InteractiveDocker` otherwise you'll execute only part of the code you wrote:
```python
container = InteractiveAsync...execute()
container.cmd("echo aaa")
```
will almost always fail as docker discards the input at some point during the initialization (bash does the same, so it's most probably TTY feature).
```python
container = InteractiveAsync...execute()
container.is_responsive()
container.cmd("echo aaa")
```
Works fine as `is_responsive` waits until the first correct answer. And this is necessarily even when you execute `docker run` directly without the underlying bash (even without tty)