test_search_paid_app

https://github.com/mozilla/gaia-ui-tests/issues/561

## Precodition
1. Marketplace-dev app is installed on the test Firefox OS device.
2. Marketplace-dev app has been successfully launched.


## Steps to test
- [x]  Do a blank search from the marketplace homepage
- [x]  Click on Filter link in the header on the search results page
- [x] Choose the option to filter for "paid" apps and click "Apply"
- [ ] Click on any of the search results

## Expected result:
- [x]  Search returns only paid apps.
- [ ]  Clicking on any of the search results loads the details page for that paid app.