test,vm: enable strict mode for vm tests

Some vm tests are not in strict mode because they need to create and use
global variables. By using `global.foo` instead of just `foo`, we can
still enable strict mode.

PR-URL: https://github.com/nodejs/node/pull/6209


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>
Reviewed-By: Santiago Gimeno <santiago.gimeno@gmail.com>
Reviewed-By: Minwoo Jung <jmwsoft@gmail.com>