Enable frozen string for spec/mailers

Partially addresses:

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

spec/mailers/abuse_report_mailer_spec.rb

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

spec/mailers/emails/merge_requests_spec.rb

+# frozen_string_literal: true
+
 require 'spec_helper'
 require 'email_spec'
 

spec/mailers/emails/pages_domains_spec.rb

+# frozen_string_literal: true
+
 require 'spec_helper'
 require 'email_spec'
 

spec/mailers/emails/profile_spec.rb

+# frozen_string_literal: true
+
 require 'spec_helper'
 require 'email_spec'
 

spec/mailers/notify_spec.rb

+# frozen_string_literal: true
+
 require 'spec_helper'
 require 'email_spec'
 

spec/mailers/repository_check_mailer_spec.rb

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