spec the new behavior of .class_for

and more robustly spec the ancestor behavior

spec/policies/base_policy_spec.rb

 require 'spec_helper'
 
 describe BasePolicy, models: true do
-  let(:build) { Ci::Build.new }
-
   describe '.class_for' do
     it 'detects policy class based on the subject ancestors' do
-      expect(described_class.class_for(build)).to eq(Ci::BuildPolicy)
+      expect(described_class.class_for(GenericCommitStatus.new)).to eq(CommitStatusPolicy)
     end
 
     it 'detects policy class for a presented subject' do
-      presentee = Ci::BuildPresenter.new(build)
+      presentee = Ci::BuildPresenter.new(Ci::Build.new)
 
       expect(described_class.class_for(presentee)).to eq(Ci::BuildPolicy)
     end
+
+    it 'uses GlobalPolicy when :global is given' do
+      expect(described_class.class_for(:global)).to eq(GlobalPolicy)
+    end
   end
 end