Options for implementing test coverage, CreateReactApp.

Currently, The cliff-effects group is in favor of option number 2.

Auxiliary wiki page, see Guide Code-Coverage-with-React-Scripts-test for overview

1. Change package.json test script completely. #634

// package.json add test script;
"test": "react-scripts test --coverage",
// .travis.yml
npm install -g codecov
npm test && codecov

2. Add coverage script and change test script in .travis.yml #632

// package.json add coverage script;
"coverage": "react-scripts test --coverage",
// .travis.yml
npm install -g codecov
npm test --coverage && codecov

3. Only change test script in .travis.yml #631

// .travis.yml
npm install -g codecov
npm test --coverage && codecov