test: fix V8 test on big-endian machines

Ref: https://github.com/nodejs/node/pull/12143#issuecomment-291131159
PR-URL: https://github.com/nodejs/node/pull/12186


Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>