Use new RailsHelpers module

spec/lib/gitlab/favicon_spec.rb

 require 'rails_helper'
 
 RSpec.describe Gitlab::Favicon, :request_store do
+  include RailsHelpers
+
   describe '.main' do
     it 'defaults to favicon.png' do
-      allow(Rails).to receive(:env).and_return(ActiveSupport::StringInquirer.new('production'))
+      stub_rails_env('production')
       expect(described_class.main).to match_asset_path '/assets/favicon.png'
     end
 
     it 'has blue favicon for development', unless: Gitlab.ee? do
-      allow(Rails).to receive(:env).and_return(ActiveSupport::StringInquirer.new('development'))
+      stub_rails_env('development')
       expect(described_class.main).to match_asset_path '/assets/favicon-blue.png'
     end
 
@@ -24,7 +26,7 @@ RSpec.describe Gitlab::Favicon, :request_store do
 
     context 'asset host' do
       before do
-        allow(Rails).to receive(:env).and_return(ActiveSupport::StringInquirer.new('production'))
+        stub_rails_env('production')
       end
 
       it 'returns a relative url when the asset host is not configured' do

spec/lib/gitlab_spec.rb

 # frozen_string_literal: true
 
-require 'fast_spec_helper'
-
-require_dependency 'gitlab'
+require 'spec_helper'
 
 describe Gitlab do
+  include RailsHelpers
+
   describe '.root' do
     it 'returns the root path of the app' do
       expect(described_class.root).to eq(Pathname.new(File.expand_path('../..', __dir__)))
@@ -113,7 +113,7 @@ describe Gitlab do
 
     it 'is true when dev env' do
       allow(described_class).to receive_messages(com?: false, org?: false)
-      allow(Rails).to receive(:env).and_return(ActiveSupport::StringInquirer.new('development'))
+      stub_rails_env('development')
 
       expect(described_class.dev_env_org_or_com?).to eq true
     end