test: explicitly set global in test-repl

The test intentionally assigns a global. Use `global` namespace to make
it clear that it is intentional and not an accidental leak.

PR-URL: https://github.com/nodejs/node/pull/6026
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

test/parallel/test-repl.js

@@ -24,7 +24,7 @@ var moduleFilename = require('path').join(common.fixturesDir, 'a');
 console.error('repl test');
 
 // function for REPL to run
-invoke_me = function(arg) {
+global.invoke_me = function(arg) {
   return 'invoked ' + arg;
 };