test: test buffer behavior when zeroFill undefined

When ArrayBufferAllocator has an undefined zeroFill property,
Buffer.allocUnsafe() should zero fill.

Refs: https://github.com/nodejs/node/commit/27e84ddd4e1#commitcomment-19182129

PR-URL: https://github.com/nodejs/node/pull/11706


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Franziska Hinkelmann <franziska.hinkelmann@gmail.com>