Extracted favicon_spec.rb in to ee directory

ee/spec/lib/gitlab/favicon_spec.rb

+require 'rails_helper'
+
+RSpec.describe Gitlab::Favicon, :request_store do
+  describe '.main' do
+    it 'has green favicon for development' do
+      allow(Rails).to receive(:env).and_return(ActiveSupport::StringInquirer.new('development'))
+      expect(described_class.main).to match_asset_path 'favicon-green.png'
+    end
+  end
+end