getDifferencePercent fix (#233)

[nikmazur]

PR Details

Fix to getDifferencePercent method in ImageComparisonUtil

Description

The getDifferencePercent was calculating pixels incorrectly: instead of calculating each different pixels towards the overall counter, it was using an overall sum of differences. This PR fixes that.

Related Issue

https://github.com/romankh3/image-comparison/issues/233

Motivation and Context

getDifferencePercent can be used to retrieve exact % of differences for ignoring them in ImageComparisonResult config. This can be used to individually configure tests with different images to pass.

How Has This Been Tested

I have ran and updated the existing tests, and also added a new one which gets and sets percentage of differences. I have also tested these changes in my own project, which uses this library with Selenide for Web visual testing: https://github.com/nikmazur/ui-visual-testing

Types of changes

• [ ] Docs change / refactoring / dependency upgrade
• [x] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)

Checklist

• [x] My code follows the code style of this project.
• [ ] My change requires a change to the documentation.
• [ ] I have updated the documentation accordingly.
• [x] I have read the CONTRIBUTING document.
• [x] I have added tests to cover my changes.
• [x] All new and existing tests passed.