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)

Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

tests/ui-regression/package.json

@@ -12,7 +12,7 @@
         "fs": "0.0.1-security",
         "mocha": "^5.2.0",
         "mocha-json-report": "0.0.2",
-        "pixelmatch": "^4.0.2",
+        "pixelmatch": "^5.0.2",
         "png-js": "^0.1.1",
         "pngjs2": "^2.0.0",
         "polyserve": "^0.27.13",