run-bats.sh's new bounded parallel dispatcher used nproc/wait -n,
which are bash 4.3+/GNU-only and silently drop the concurrency cap on
macOS's stock bash 3.2 (the wait -n error is swallowed by `|| true`).
Its sibling tests/run-tests.sh, changed in the same PR and explicitly
bash-3.2-safe, already solves this with getconf + a batched wait.
Ported that same pattern here for consistency and to actually meet the
compatibility goal.
Refs: #95
Defame1297
4003c6a273