When unit test fails, core file is created. Kyua's 'debug' command can run GDB on it and provide a backtrace. Unfortunately Kyua is picky about location of these core files:

Attempting core file candidate /tmp/kyua.J8TrsX/131/work/query_test.core: not found
Attempting core file candidate /home/newman/isc/ws/bind9/lib/ns/tests/query_test.core: not found
Attempting core file candidate /tmp/kyua.J8TrsX/131/work/core.271745: not found
Attempting core file candidate /cores/core.271745: not found

Instead of Kyua being configured to match system core file handler, one has to change system core file handler to be in line with Kyua's expectations.

The safest location to put core files in is /cores/, name of the core file needs to be core.<PID>, see https://gitlab.isc.org/isc-private/devops/issues/69.