Fix tests being broken because of different default setting

spec/controllers/api/v1/timelines/public_controller_spec.rb

@@ -44,6 +44,10 @@ describe Api::V1::Timelines::PublicController do
   context 'without a user context' do
     let(:token) { Fabricate(:accessible_access_token, resource_owner_id: nil) }
 
+    before do
+      Setting.timeline_preview = true
+    end
+
     describe 'GET #show' do
       it 'returns http success' do
         get :show