Sanity tests for Consumer Pages

This PR is for issue https://github.com/mozilla/marketplace-tests/issues/524

Tests are strengthened/modified, new tests added to address https://moztrap.mozilla.org/runtests/run/57/env/112/ list with sanity tests
In `category.py` you use `is_element_visible()` to check for visibility and in this file you use `is_displayed()`. Please be consistent and use the same method everywhere. `is_element_visible()` seems like it would be preferable as it abstracts away `is_displayed()` and also deals with `NoSuchElementException` and `ElementNotVisibleException`.