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

.cirrus.yml

@@ -226,7 +226,10 @@ task:
   env:
     CPUS: 12 # always get that much for cirrusci macOS instances
     BUILD_JOBS: $CPUS
-    TEST_JOBS: $CPUS # already fast enough to not be worth tuning
+    # Test performance regresses noticably when using all cores. 8 seems to
+    # work OK. See
+    # https://postgr.es/m/20220927040208.l3shfcidovpzqxfh%40awork3.anarazel.de
+    TEST_JOBS: 8
 
     CIRRUS_WORKING_DIR: ${HOME}/pgsql/
     CCACHE_DIR: ${HOME}/ccache