fetch plugin test: minor indentation fix

__tests__/plugins/fetch.test.js

@@ -3,7 +3,7 @@ const makeServiceWorkerEnv = require('service-worker-mock');
 global.fetch = require('node-fetch');
 jest.mock('node-fetch', () => {
   const context = {
-        then: jest.fn().mockImplementationOnce(() => {
+    then: jest.fn().mockImplementation(() => {
       const response = { test: "success" };
       return Promise.resolve(response);
     })
@@ -34,6 +34,7 @@ describe("plugin: fetch", () => {
     require("../../plugins/fetch.js");
 
     const returnedJSON = await self.LibResilientPlugins[0].fetch('https://resilient.is/test.json');
+    expect(fetch).toHaveBeenCalled();
     expect(returnedJSON).toEqual({test: "success"})
   });
 });