test: make test-v8-serdes work without stdin

If stdin was closed or referred to a file, this didn't work, because it was accessed via file descriptor.

Instead, use another generic native object.

cherry-picked from ayojs/ayo#63

Checklist

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

Affected core subsystem(s)

test