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