Bug 1114640: Create a new test case for the rating feature of the Music App

To me, it's better to define a locator function like we do [in the calendar](https://github.com/mozilla-b2g/gaia/blob/master/tests/python/gaia-ui-tests/gaiatest/apps/calendar/app.py#L164). This has an advantage: you don't have to repeat `By.CSS_SELECTOR` and call the replace function each time you want to use a given selector.

Also, if you want to replace a value in a string, the convention is (in Python 2) to use [the string formatting operation](https://docs.python.org/2/library/stdtypes.html#string-formatting). Here, it would be:
``` Python
'.rating-star[data-rating="%s"]' % rating
```