Add rating out of 5 render helpers and test validators

:clipboard: **Description**

Add validators for
- integer rating out of 5
- float rating out of 5

to `test-validators` and use them consistently in rating badge service tests

Refs https://github.com/badges/shields/pull/4898#discussion_r407105049

Start by locating the test-validators helpers and the rating badge service tests. Check how existing integer and float rating validators are structured, then add the out-of-5 cases and use them consistently in the rating badge service tests. Done means both rating forms are covered and the affected test suite passes.

Written by the indexing model from the issue text.