test: repl tab completion test

It checks that `eval` is called with `.scope` as an input string.

PR-URL: https://github.com/nodejs/node/pull/5534


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>