Changes

testenv: support fetching coredumps in jenkins

The Osmocom jenkins nodes run inside LXCs. When we get a coredump it
appears on the host, fetch it from there via testenv-coredump-helper,
which gets added to the hosts in the related patch.

Related: osmo-ci I7e66c98106b7028a393e3b873e96ae2dcb412c48
Related: OS#6769
Change-Id: I3784b4cbcef08b26f77b6f6f7a70a830d9c81a18