refactor: explicit mocking for analytics (#41562)

The previous approach did avoid a fair number of jest.mock calls, but
made debugging the tests harder. If you don't know about the mapping
it's unclear why the imported module does not behave as normal.

By forcing the use of jest.mock it means that the answer to that
question is in the test you are working on.

client/src/redux/ga-saga.test.js

@@ -1,8 +1,12 @@
+/* global jest */
+
 import { types } from '.';
 import { createGaSaga } from './ga-saga';
 import ga from '../analytics';
 import { expectSaga } from 'redux-saga-test-plan';
 
+jest.mock('../analytics');
+
 describe('ga-saga', () => {
   it('calls GA after executeGA action', () => {
     const GaTypes = { event: ga.event, page: ga.pageview, modal: ga.modalview };