Removing core dumps from testbots between runs

When a test run results in a fatal error, DrupalCI captures the core dump on the testbot for further analysis. However, if the testbot is long lived, any prior core dumps from previous runs remain on the bot. We'll be updating core dump handling to remove core dumps from the bot between runs.

Production outage: No
Change affects: DrupalCI artifacts
Deployment schedule: week of 2017-08-21
Backout plan: revert or fix in place