Merge bitcoin/bitcoin#31045: ci: Add missing -DWERROR=ON to test-each-commit

fa1cffacae ci: Install missing nproc in macos task (MarcoFalke)
faf7a2bccc ci: Add missing -DWERROR=ON to test-each-commit (MarcoFalke)

Pull request description:

  Found by in Sjors in https://github.com/bitcoin/bitcoin/pull/27260#discussion_r1785860610 (Thanks!)

  Also, includes an unrelated commit to simplify the ci scripts, by assuming nproc exists on macos as well. (Having more than one commit is also required to trigger the `test-each-commit` task)

ACKs for top commit:
  theuni:
    utACK fa1cffacae.

Tree-SHA512: 881df6964da31c3f7161b763bdf74029743567272c7a4db24ded10909a8cedad39b494a9f486f0b059bbeb0beb190dcd91369650d014e1fc3b5f7ae07cd54771

ci/test/03_test_script.sh

@@ -13,12 +13,11 @@ export LSAN_OPTIONS="suppressions=${BASE_ROOT_DIR}/test/sanitizer_suppressions/l
 export TSAN_OPTIONS="suppressions=${BASE_ROOT_DIR}/test/sanitizer_suppressions/tsan:halt_on_error=1"
 export UBSAN_OPTIONS="suppressions=${BASE_ROOT_DIR}/test/sanitizer_suppressions/ubsan:print_stacktrace=1:halt_on_error=1:report_error_type=1"
 
+echo "Number of available processing units: $(nproc)"
 if [ "$CI_OS_NAME" == "macos" ]; then
   top -l 1 -s 0 | awk ' /PhysMem/ {print}'
-  echo "Number of CPUs: $(sysctl -n hw.logicalcpu)"
 else
   free -m -h
-  echo "Number of CPUs (nproc): $(nproc)"
   echo "System info: $(uname --kernel-name --kernel-release)"
   lscpu
 fi