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.

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• commit message follows commit guidelines

Affected core subsystem(s)

test