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/mailers/user_notifications_spec.rb

@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 require "rails_helper"
 
 describe UserNotifications do
@@ -180,7 +182,7 @@ describe UserNotifications do
       it "excludes posts that are newer than editing grace period" do
         SiteSetting.editing_grace_period = 5.minutes
         too_new = Fabricate(:topic, user: Fabricate(:user), title: "Oops I need to edit this", created_at: 1.minute.ago)
-        too_new_post = Fabricate(:post, user: too_new.user, topic: too_new, score: 100.0, post_number: 1, created_at: 1.minute.ago)
+        _too_new_post = Fabricate(:post, user: too_new.user, topic: too_new, score: 100.0, post_number: 1, created_at: 1.minute.ago)
         html = subject.html_part.body.to_s
         expect(html).to_not include too_new.title
       end
@@ -767,7 +769,7 @@ describe UserNotifications do
 
       context "when customized" do
         let(:custom_body) do
-          body = <<~BODY
+          body = +<<~BODY
             You are now officially notified.
             %{header_instructions}
             %{message} %{respond_instructions}