Use sinon for mocking axios

[hdurix]

I think we should sinon rather than jest for mocking axios in unit tests.

It's really hard to mock properly axios with the couple Jest & TypeScript. For now, we have something like:

import axios from 'axios';

const mockedAxios: any = axios;

jest.mock('axios', () => ({
  get: jest.fn(),
  put: jest.fn()
}));

mockedAxios.get.mockReturnValue(Promise.resolve({}));

But as it's cast as any, it's not really TypeScript friendly...

It's how it is done in react tests: https://github.com/jhipster/generator-jhipster/blob/master/generators/client/templates/react/src/test/javascript/spec/app/shared/reducers/application-profile.spec.ts.ejs#L79.