Kill dangling test processes before running tests

Kill dangling test processes before running tests

Tests often fail because of "dangling" lttng processes left on CI slaves. I am not sure what leaks them (either failing tests or cancelled jobs), but they are a very frequent cause of build failures.

The code is this diff is taken from lttng-tools warn_processes.sh.

We could also clean-up at the end of the job, but I am not sure how to get Jenkins to run a "clean-up" when a job is cancelled (e.g. manually or after a time-out).