tests: Try to use coredumpctl to extract core files.

tests: Try to use coredumpctl to extract core files.

If systemd-coredump is installed we have to use coredumpctl to extract
the core file to test. Unfortunately systemd-coredump/coredumpctl seem
to be somewhat fragile if multiple core dumps are generated/extracted
at the same time. So use a lock file to only run one core dump test at
a time (under make -j).

Signed-off-by: Mark Wielaard <mark@klomp.org>