make test: kill all remaining subprocesses on exit

This change introduces a wrapper script which kills all processes in
the same process group as itself (with the exception of the script).
Using this script to run the unit tests should prevent stale processes
left behind in some cases (e.g. when test framework crashes).

Change-Id: If3b9201c06b87fa6be095721436893207d09b5e4
Signed-off-by: Klement Sekera <ksekera@cisco.com>