check apt::source in catalog, not the file it creates

We should trust the apt module to do the right thing,
and just test that apt::source is part of the catalogue.

spec/classes/package_spec.rb

@@ -10,7 +10,7 @@
 
   shared_examples 'debian' do |operatingsystem, lsbdistcodename|
     let(:facts) {{ :operatingsystem => operatingsystem, :osfamily => 'Debian', :lsbdistcodename => lsbdistcodename }}
-    it { should contain_file('/etc/apt/sources.list.d/nginx.list') }
+    it { should contain_apt__source('nginx') }
   end
 
   shared_examples 'suse' do |operatingsystem|