ci: macos: Reduce test concurrency
Test performance regresses noticably when using all cores. This is more
pronounced with meson than with autoconf, presumably because meson will
schedule the "full number" of tests more consistently. 8 seems to work
OK.
Discussion: https://postgr.es/m/
20220927040208.l3shfcidovpzqxfh@awork3.anarazel.de
Backpatch: 15-, where CI was introduced