abuild.conf: set CTEST_PARALLEL_LEVEL=$JOBS
same as CARGO_BUILD_JOBS etc, else it defaults to 1. theoretically this could break some very poorly written test suite, but gentoo has set it since 2016 with no apparent issues.
https://cgit.gentoo.org/repo/gentoo.git/commit/?id=adfccec02c2474a509fc10d580a25d72f33cab55