DEV: use #frozen_string_literal: true on all spec

This change both speeds up specs (less strings to allocate) and helps catch
cases where methods in Discourse are mutating inputs.

Overall we will be migrating everything to use #frozen_string_literal: true
it will take a while, but this is the first and safest move in this direction

spec/models/draft_sequence_spec.rb

@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 require 'rails_helper'
 
 describe DraftSequence do