test: prepare test-hash-seed for CI

Reduce the time it takes to run test/pummel/test-hash-seed by switching
from spawnSync() to spawn(). On my computer, this reduces the runtime
from about 80 seconds to about 40 seconds. This test is not (yet) run
regularly on CI, but when it was run recently, it timed out.

PR-URL: https://github.com/nodejs/node/pull/25522


Reviewed-By: Ali Ijaz Sheikh <ofrobots@google.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Minwoo Jung <minwoo@nodesource.com>