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>