Provide a way to specify test environment setup file for Jest

Created by: gaelollivier

My App is using localStorage but when I run tests, I get this error:

ReferenceError: localStorage is not defined

Should we provide a polyfill to be able to mock it with Jest ?

Workaround

Add a --setupTestFrameworkScriptFile ./localStoragePolyfill.js to the test command in package.json, where ./localStoragePolyfill looks like this:

const localStorageMock = (() => {
  let store = {}
  return {
    getItem(key) {
      return store[key]
    },
    setItem(key, value) {
      store[key] = value.toString()
    },
    clear() {
      store = {}
    }
  };
})()

global.localStorage = localStorageMock

I guess it should be added to config/polyfills.js.

I can PR if that's fine. Not sure if we should use a simple inline polyfill or use something like node-localstorage though.