Implement highscore/leaderboard top 10 best games and add tests with jest

package.json

@@ -7,7 +7,7 @@
     "dev": "webpack-dev-server --mode development --open",
     "build": "webpack --optimize-minimize --config webpack.production.config.js",
     "webpack": "webpack --config webpack.config.js --watch",
-    "test": "echo \"Error: no test specified\" && exit 1"
+    "test": "jest"
   },
   "keywords": [],
   "author": "",
@@ -20,6 +20,7 @@
     "@babel/preset-react": "^7.0.0",
     "babel-loader": "^8.0.6",
     "babel-plugin-styled-components": "^1.10.6",
+    "jest": "^24.9.0",
     "react-hot-loader": "^4.12.11",
     "webpack": "^4.39.2",
     "webpack-cli": "^3.3.7",