try adding support for jest

[norbertschuler]

Fixes #1679 and #1302

Changes in this pull request:

• trying to added support for jest with running npm run test or jest (after having installed jest globally with npm install jest --global)

Problem:

 FAIL  __tests__/App-test.js
  ● Test suite failed to run

    Cannot find module 'warnOnce' from 'react-native-implementation.js'
      
      at Resolver.resolveModule (node_modules/jest/node_modules/jest-resolve/build/index.js:169:17)
      at Object.<anonymous> (node_modules/react-native/Libraries/react-native/react-native-implementation.js:14:18)

I tried some stuff from jestjs/jest#6933 but did not succeed.

[norbertschuler]

I create an alternate configuration not using babel7-jest, but it also does not work: https://github.com/Plant-for-the-Planet-org/treecounter-app/tree/feature/jest-alternative