add script for code coverage

It is not easy to remember --watchAll=false

https://github.com/facebook/create-react-app/issues/6888

package.json

@@ -43,6 +43,7 @@
         "start": "react-scripts start",
         "build": "react-scripts build",
         "test": "react-scripts test --env=./test-env.js",
+        "coverage": "react-scripts test --env=./test-env.js --coverage --watchAll=false",
         "eject": "react-scripts eject",
         "lint": "tsc --noEmit && eslint '*/**/*.{js,ts,tsx}' --quiet --fix"
     },