Refactor Todos feature spec to be faster and less brittle

We now only create two Todos instead of 21 when testing pagination, and
we've updated the test to be less brittle when dealing with slower CI
environments.

spec/features/todos/todos_spec.rb

 require 'spec_helper'
 
 describe 'Dashboard Todos', feature: true do
-  let(:user){ create(:user) }
-  let(:author){ create(:user) }
-  let(:project){ create(:project) }
-  let(:issue){ create(:issue) }
-  let(:todos_per_page){ Todo.default_per_page }
-  let(:todos_total){ todos_per_page + 1 }
+  let(:user)    { create(:user) }
+  let(:author)  { create(:user) }
+  let(:project) { create(:project) }
+  let(:issue)   { create(:issue) }
 
   describe 'GET /dashboard/todos' do
     context 'User does not have todos' do
@@ -46,31 +44,35 @@ describe 'Dashboard Todos', feature: true do
     end
 
     context 'User has multiple pages of Todos' do
-      let(:todo_total_pages){ (todos_total.to_f/todos_per_page).ceil }
-
       before do
-        todos_total.times do
-          create(:todo, :mentioned, user: user, project: project, target: issue, author: author)
-        end
+        allow(Todo).to receive(:default_per_page).and_return(1)
+
+        # Create just enough records to cause us to paginate
+        create_list(:todo, 2, :mentioned, user: user, project: project, target: issue, author: author)
 
         login_as(user)
-        visit dashboard_todos_path
       end
 
       it 'is paginated' do
+        visit dashboard_todos_path
+
         expect(page).to have_selector('.gl-pagination')
       end
 
       it 'is has the right number of pages' do
-        expect(page).to have_selector('.gl-pagination .page', count: todo_total_pages)
+        visit dashboard_todos_path
+
+        expect(page).to have_selector('.gl-pagination .page', count: 2)
       end
 
-      describe 'deleting last todo from last page', js: true do
+      describe 'completing last todo from last page', js: true do
         it 'redirects to the previous page' do
-          page.within('.gl-pagination') do
-            click_link todo_total_pages.to_s
-          end
-          first('.done-todo').click
+          visit dashboard_todos_path(page: 2)
+          expect(page).to have_content(Todo.first.body)
+
+          click_link('Done')
+
+          expect(current_path).to eq dashboard_todos_path
           expect(page).to have_content(Todo.last.body)
         end
       end