Fixed guard breakage in testem.json

Fixed broken **testem** *guard* capability by restoring ```js/tests/tests/**/*.coffee``` to ```src_files``` section.

Note that, even though we don't have any ```.js``` files in the ```src_files``` section, we still look for them with the entry on line 15:

        "js/tests/tests/**/*.js"

and we still run tests on them with

        "js/tests/tests/**/*.js"

on line 26.