Update pixelmatch requirement in /tests/ui-regression

Updates the requirements on [pixelmatch](https://github.com/mapbox/pixelmatch) to permit the latest version.
- [Release notes](https://github.com/mapbox/pixelmatch/releases)
- [Commits](https://github.com/mapbox/pixelmatch/compare/v4.0.2...v5.0.2

)

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