test: change buffer offset to accommodate V8 BackingStore

To avoid problem with the behavior of new V8 BackingStore API,
change the offset. The base address of each test case will be
different.

Fixes: https://github.com/nodejs/node/issues/31061

PR-URL: https://github.com/nodejs/node/pull/31171


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: David Carlier <devnexen@gmail.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>