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: nodejs#548
PR-URL: nodejs#5528
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Wyatt Preul <wpreul@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>

test/known_issues/test-vm-function-redefinition.js

@@ -0,0 +1,11 @@
+'use strict';
+// Refs: https://github.com/nodejs/node/issues/548
+require('../common');
+const assert = require('assert');
+const vm = require('vm');
+const context = vm.createContext();
+
+vm.runInContext('function test() { return 0; }', context);
+vm.runInContext('function test() { return 1; }', context);
+const result = vm.runInContext('test()', context);
+assert.strictEqual(result, 1);