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>
Why would you always like to implement these basic functions. There are 6601 commands/programs on my Fedora and IMO it's better to execute them all the same way:
```python
self.cmd("echo hallo")
self.cmd("touch /this")
self.cmd("exit")
```

than use:
```python
self.echo("hallo")
self.cmd("touch /this")
self.exit()
```

Plus this function would be useless in this context as I'm not executing echo, I'm just putting few strings (`send` vs. `sendline`) into first session. The whole command is compound during the whole iteration using multiple session, that's where it got the name from.