Enable frozen string for spec/initializers

Partially addresses:

https://gitlab.com/gitlab-org/gitlab/issues/27703

spec/initializers/6_validations_spec.rb

+# frozen_string_literal: true
+
 require 'spec_helper'
 require_relative '../../config/initializers/6_validations.rb'
 

spec/initializers/action_mailer_hooks_spec.rb

+# frozen_string_literal: true
+
 require 'spec_helper'
 
 describe 'ActionMailer hooks' do

spec/initializers/asset_proxy_setting_spec.rb

+# frozen_string_literal: true
+
 require 'spec_helper'
 
 describe 'Asset proxy settings initialization' do

spec/initializers/attr_encrypted_no_db_connection_spec.rb

+# frozen_string_literal: true
+
 require 'spec_helper'
 
 describe 'GitLab monkey-patches to AttrEncrypted' do

spec/initializers/direct_upload_support_spec.rb

+# frozen_string_literal: true
+
 require 'spec_helper'
 
 describe 'Direct upload support' do

spec/initializers/doorkeeper_spec.rb

+# frozen_string_literal: true
+
 require 'spec_helper'
 require_relative '../../config/initializers/doorkeeper'
 

spec/initializers/fog_google_https_private_urls_spec.rb

+# frozen_string_literal: true
+
 require 'spec_helper'
 
 describe 'Fog::Storage::GoogleXML::File', :fog_requests do

spec/initializers/rest-client-hostname_override_spec.rb

+# frozen_string_literal: true
+
 require 'spec_helper'
 
 describe 'rest-client dns rebinding protection' do

spec/initializers/secret_token_spec.rb

+# frozen_string_literal: true
+
 require 'spec_helper'
 require_relative '../../config/initializers/01_secret_token'
 

spec/initializers/settings_spec.rb

+# frozen_string_literal: true
+
 require 'spec_helper'
 require_relative '../../config/initializers/1_settings' unless defined?(Settings)
 

spec/initializers/trusted_proxies_spec.rb

+# frozen_string_literal: true
+
 require 'spec_helper'
 
 describe 'trusted_proxies' do

spec/initializers/zz_metrics_spec.rb

+# frozen_string_literal: true
+
 require 'spec_helper'
 
 describe 'instrument_classes' do