test: fix allocUnsafe uninitialized buffer check

Fixes parallel/test-buffer-bindingobj-no-zerofill to properly check
that buffers created with `Buffer.allocUnsafe()` are not zero-filled.

The test introduced in #11706 passes even if the buffer has been
zero-filled and fails if none of the buffer values are zero.

Refs: https://github.com/nodejs/node/pull/11706
PR-URL: https://github.com/nodejs/node/pull/12290


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>