test: bug repro for vm function redefinition

This commit adds a failing test case for the vm module.
Currently, if runInContext() defines a function, and a later call
to runInContext() redefines the same function, the original
function is not overwritten.

Refs: https://github.com/nodejs/node/issues/548
PR-URL: https://github.com/nodejs/node/pull/5528


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Wyatt Preul <wpreul@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>