Merge queue wall time is dominated by unit-test-zstd on rhel80 (about an hour for the full Python suite with --zstd). After unit-test-hook-tiered is removed, that is the remaining task that keeps merge queue well above 20 minutes.

Split it into four tasks with run.py -b K/4, the same batching used by unit-test-bucket00 and the other Python buckets. Merge queue selects this task by name, so GitHub required checks need to follow the new bucket names.