Enable frozen string for spec/*.rb

Partially addresses:

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

spec/factories_spec.rb

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

spec/fast_spec_helper.rb

+# frozen_string_literal: true
+
 require 'bundler/setup'
 
 ENV['GITLAB_ENV'] = 'test'

spec/rake_helper.rb

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

spec/simplecov_env.rb

+# frozen_string_literal: true
+
 require 'simplecov'
 require 'active_support/core_ext/numeric/time'
 require_relative '../lib/gitlab/utils'

spec/spec_helper.rb

+# frozen_string_literal: true
+
 require './spec/simplecov_env'
 SimpleCovEnv.start!