Make fixtures/u2f.rb deterministic

spec/javascripts/fixtures/u2f.rb

@@ -3,7 +3,7 @@ require 'spec_helper'
 context 'U2F' do
   include JavaScriptFixturesHelpers
 
-  let(:user) { create(:user, :two_factor_via_u2f) }
+  let(:user) { create(:user, :two_factor_via_u2f, otp_secret: 'otpsecret:coolkids') }
 
   before(:all) do
     clean_frontend_fixtures('u2f/')
@@ -33,6 +33,7 @@ context 'U2F' do
 
     before do
       sign_in(user)
+      allow_any_instance_of(Profiles::TwoFactorAuthsController).to receive(:build_qr_code).and_return('qrcode:blackandwhitesquares')
     end
 
     it 'u2f/register.html.raw' do |example|