Enable frozen string for spec/lib/gitlab/i18n

Partially addresses:

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

spec/lib/gitlab/i18n/metadata_entry_spec.rb

+# frozen_string_literal: true
+
 require 'spec_helper'
 
 describe Gitlab::I18n::MetadataEntry do

spec/lib/gitlab/i18n/po_linter_spec.rb

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

spec/lib/gitlab/i18n/translation_entry_spec.rb

+# frozen_string_literal: true
+
 require 'spec_helper'
 
 describe Gitlab::I18n::TranslationEntry do