Ratio component – Improve code comments, clarity of visual tests.

(brief description / PR title)

Status: **Ready to merge**

Reviewers: @kpeatt @mobify-derrick 
Ticket: #44

## Changes
- Improve code comments for Ratio component
- Improve descriptions for each test
- Replace visually confusing fixture html and svg files with simpler ones (that are more clearly placeholders).

## How to test-drive this PR
- Review diff
- View ratio test html file and check test descriptions to ensure it’s easy to tell what the result should look like, i.e. can you tell whether a given test case passes or fails.