以下のドキュメントを参考にしつつ
https://redux.js.org/recipes/writing-tests/#action-creators
http://www.wheresrhys.co.uk/fetch-mock/#usageusage-with-jest

jestの設定ファイルに


jest.mock('node-fetch', () => require('fetch-mock-jest').sandbox());

テストファイルにはfetchMockをセット。


import * as actions from '../fetchActions';
import configureMockStore from 'redux-mock-store';
import thunk from 'redux-thunk';

const middlewares = [thunk];
const mockStore = configureMockStore(middlewares);
const fetchMock = require('node-fetch');


it('should create GET_USER_SUCCESS when user fetching is done', function() {
  fetchMock.getOnce('path:/user', {
    body: { user: { name: 'ABC', id: 123 } },
  });

  const expectedActions = [
    { type: actionTypes.GET_USER },
    { type: actionTypes.GET_USER_SUCCESS,
      data: {
        user: { name: 'ABC', id: 123 },
      },
      status: 200,
    }
  ];

  return store.dispatch(actions.getUser()).then(() => {
    // return of async actions
    expect(store.getActions()).toEqual(expectedActions);
  });
});

isomorphic-fetchを使用している場合、上記のnode-fetchをisomorphic-fetchに置き換えてテスト可能。