Fix system test backtrace generation on OpenBSD

On Linux core dump contains absolute path to crashed binary

Core was generated by `/home/newman/isc/ws/bind9/bin/named/.libs/lt-named -D glue-ns1 -X named.lock -m'.

However, on OpenBSD there's only a basename

Core was generated by `named'.

This commit adds support for the latter, retains the former.

In https://gitlab.isc.org/isc-projects/bind9/-/pipelines/54498 I crashed several named instances and checked that we still match crashed binary on the rest of platforms.

https://gitlab.isc.org/isc-projects/images/-/merge_requests/84 is required to be merged first.