fix: use sysctl to find cpu cores on macos
fix: use sysctl to find cpu cores on macos

Newer versions of macos seems to lack `nproc` (at least for me),
so switch to using `sysctl` instead.

Also, add `DEBUG` as an option to the test runner.

Fixes: https://github.com/jbergstroem/mariadb-alpine/issues/112

jbergstroem/mariadb-alpine