subtests.docker_cli: Add --sig-proxy tests

Hi guys,

this pull request changes the old kill test to make it possible to run some of it's tests using `os.kill` on `docker run` and `docker attach`, thus test the `--sig-proxy=true` on already existing tests.

Additionally it adds basic test, which verifies `--sig-proxy` works how it should (booth, positive and negative tests).

The first commit modifies the `OutputGood` to log the command output in case of failure. This is really essential for log analysis.

Regards,
Lukáš
``negative_test`` (and maybe others) could probably just be removed, since they'll very rarely (if ever) be changed.  You could make them into class attributes, and then also save yourself from documenting them in one-stroke.  Just because something *could* be an option, doesn't mean it should be :grinning: If it becomes important later, it can always be added, but adding prematurely just means more work for yourself and whomever runs the test.