subtests.docker_cli: Add logs_follow test

This test uses `docker logs --follow` and checks it works correctly. (Issue https://github.com/autotest/autotest-docker/issues/250 )

Signed-off-by: Lukáš Doktor <ldoktor@redhat.com>
I had to look up what R0903 was, okay, this is fine.

FYI:   It's generally more helpful to others to comment **why** instead of **what**.  "Only containment" describes *what*, which can be inferred by examining the code.  Understanding *why* requires much more extensive *usage* examination, so having a comment about it saves considerable time.  For example, here you could say instead:

    #  Allow test code to incrementally examine output of different instances in a common way.