Changing path of site.json file to the new system folder

Updating the path of the site.json file based on changes to the paths in adaptive.js.

Status: **Ready for merge**

Reviewers: @mobify-derrick @marlowpayne @jansepar 

## Changes
- changed the path in preview.js to use the new test folder in adaptive.js

## How to test-drive this PR
- generate a new adaptive.js project 
- install nightwatch-commands from this branch
- run ```grunt nightwatch```
- ensure tests run