[v9_11] Enhance unit test debugging
When unit test fails, core file is created. Kyua's 'debug' command can run GDB on it and provide backtrace. Unfortunately Kyua is picky about location of these core files we opt to use custom Kyua fork and copy core files from Kyua working directory to source tree and make it available in GitLab.
(cherry picked from commit 94fa27e4)