chore(jest): create axios mock

improves #26

src/app/__mocks__/axios.js

@@ -0,0 +1,9 @@
+/**
+ * @link https://medium.com/@kilgarenone/use-jest-to-test-redux-async-action-creator-with-axios-in-a-create-react-app-app-d9c9b52eba5e
+ */
+const mockAxios = jest.genMockFromModule('axios');
+
+// this is the key to fix the axios.create() undefined error!
+mockAxios.create = jest.fn(() => mockAxios);
+
+export default mockAxios;