test: fix test-vm-cached-data to work with old v8

While `let` no longer needs to run in `strict mode` in v8 5.x it throws
in v8 4. This modification will make the test-vm-cached-data work in
older version of node.

Refs: https://github.com/nodejs/node/pull/6280

PR-URL: https://github.com/nodejs/node/pull/6317


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Fedor Indutny <fedor.indutny@gmail.com>
Reviewed-By: Michaël Zasso <mic.besace@gmail.com>