Add tests for groups pages, DRY html into a component

test/javascripts/acceptance/groups-test.js.es6

@@ -11,4 +11,19 @@ test("Browsing Groups", () => {
   andThen(() => {
     ok(count('.group-members tr') > 0, "it lists group members");
   });
+
+  visit("/groups/discourse/topics");
+  andThen(() => {
+    ok(count('.user-stream .item') > 0, "it lists stream items");
+  });
+
+  visit("/groups/discourse/mentions");
+  andThen(() => {
+    ok(count('.user-stream .item') > 0, "it lists stream items");
+  });
+
+  visit("/groups/discourse/messages");
+  andThen(() => {
+    ok(count('.user-stream .item') > 0, "it lists stream items");
+  });
 });

test/javascripts/helpers/create-pretender.js.es6

@@ -170,6 +170,18 @@ export default function() {
                                            slug: request.params.slug } });
     });
 
+    this.get("/groups/discourse/topics.json", () => {
+      return response(200, fixturesByUrl['/groups/discourse/posts.json']);
+    });
+
+    this.get("/groups/discourse/mentions.json", () => {
+      return response(200, fixturesByUrl['/groups/discourse/posts.json']);
+    });
+
+    this.get("/groups/discourse/messages.json", () => {
+      return response(200, fixturesByUrl['/groups/discourse/posts.json']);
+    });
+
     this.get('/t/:topic_id/posts.json', request => {
       const postIds = request.queryParams.post_ids;
       const posts = postIds.map(p => ({id: parseInt(p), post_number: parseInt(p) }));