Add basic tests for build entity used by serialzier

spec/serializers/build_entity_spec.rb

+require 'spec_helper'
+
+describe BuildEntity do
+  subject do
+    described_class.new(build, request: double).as_json
+  end
+
+  context 'when build is a regular job' do
+    let(:build) { create(:ci_build) }
+
+    it 'contains url to build page and retry action' do
+      expect(subject).to include(:build_url, :retry_url)
+      expect(subject).not_to include(:play_url)
+    end
+  end
+
+  context 'when build is a manual action' do
+    let(:build) { create(:ci_build, :manual) }
+
+    it 'contains url to play action' do
+      expect(subject).to include(:play_url)
+    end
+  end
+end