DEV: Automatically require 'rails_helper' in all specs (#16077)

It's very easy to forget to add `require 'rails_helper'` at the top of every core/plugin spec file, and omissions can cause some very confusing/sporadic errors.

By setting this flag in `.rspec`, we can remove the need for `require 'rails_helper'` entirely.

plugins/discourse-narrative-bot/spec/discourse_narrative_bot/advanced_user_narrative_spec.rb

@@ -1,7 +1,5 @@
 # frozen_string_literal: true
 
-require 'rails_helper'
-
 RSpec.describe DiscourseNarrativeBot::AdvancedUserNarrative do
   fab!(:narrative_bot) { ::DiscourseNarrativeBot::Base.new }
   fab!(:discobot_user) { narrative_bot.discobot_user }

plugins/discourse-narrative-bot/spec/discourse_narrative_bot/new_user_narrative_spec.rb

@@ -1,7 +1,5 @@
 # frozen_string_literal: true
 
-require 'rails_helper'
-
 describe DiscourseNarrativeBot::NewUserNarrative do
   fab!(:welcome_topic) { Fabricate(:topic, title: 'Welcome to Discourse') }
   fab!(:narrative_bot) { ::DiscourseNarrativeBot::Base.new }

plugins/discourse-narrative-bot/spec/discourse_narrative_bot/store_spec.rb

@@ -1,7 +1,5 @@
 # frozen_string_literal: true
 
-require 'rails_helper'
-
 describe DiscourseNarrativeBot::Store do
   describe '.set' do
     it 'should set the right value in the plugin store' do

plugins/discourse-narrative-bot/spec/discourse_narrative_bot/track_selector_spec.rb

@@ -1,7 +1,5 @@
 # frozen_string_literal: true
 
-require 'rails_helper'
-
 describe DiscourseNarrativeBot::TrackSelector do
   let(:user) { Fabricate(:user) }
   let(:narrative_bot) { ::DiscourseNarrativeBot::Base.new }