Widget unit & e2e testing infrastructure

- Adding `mock-data.js` to provide test gadget data
- Adding initial e2e and unit test files for writing further tests
- `widget.html` updated to add e2e build block expressions for visualization api, mock data, and gadget api
- gulpfile updated to configure e2e and unit gulp tasks
- `main.js` and `spreadsheet.js` modified to correctly handle array of different params from gadget "rsparam_get" call