Fix a test that did not execute intended code path

The customized result for the first stub call was not used because a
more specific response was defined earlier using `withArgs`.

src/sidebar/services/test/api-routes-test.js

@@ -83,13 +83,18 @@ describe('APIRoutesService', () => {
       );
     });
 
-    it('retries the route fetch until it succeeds', () => {
-      window.fetch.onFirstCall().returns(httpResponse(500, null));
-      return apiRoutes.routes().then(routes => {
+    it('retries the route fetch until it succeeds', async () => {
+      window.fetch
+        .withArgs('https://annotation.service/api/')
+        .onFirstCall()
+        .returns(httpResponse(500, null));
+
+      const routes = await apiRoutes.routes();
+
+      assert.calledTwice(window.fetch);
       assert.deepEqual(routes, apiIndexResponse.links);
     });
   });
-  });
 
   describe('#links', () => {
     it('returns page links', () => {