test: fix path to module for repl test on Windows

Use path join to construct the path instead of concatenating strings.
Replace backslash with double backslash so that they are escaped
correctly in the string passed to REPL.

PR-URL: https://github.com/nodejs/node/pull/3608


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>