test: add regression test for Proxy as vm context

A Proxy context should not hide built-in global objects.
Ref: nodejs#6158

PR-URL: nodejs#6967
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>

test/parallel/test-vm-context.js

@@ -72,3 +72,7 @@ assert.throws(function() {
 }, function(err) {
  return /expected-filename.js:33:130/.test(err.stack);
 }, 'Expected appearance of proper offset in Error stack');
+
+// https://github.com/nodejs/node/issues/6158
+ctx = new Proxy({}, {});
+assert.strictEqual(typeof vm.runInNewContext('String', ctx), 'function');