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.

spec/integrity/coding_style_spec.rb

@@ -1,7 +1,5 @@
 # frozen_string_literal: true
 
-require "rails_helper"
-
 def list_files(base_dir, pattern = '*')
   Dir[File.join("#{base_dir}", pattern)]
 end

spec/integrity/common_mark_spec.rb

@@ -1,6 +1,4 @@
 # frozen_string_literal: true
-require 'rails_helper'
-
 describe "CommonMark" do
   it 'passes spec' do
 

spec/integrity/i18n_spec.rb

@@ -1,6 +1,5 @@
 # frozen_string_literal: true
 
-require "rails_helper"
 require "i18n/duplicate_key_finder"
 
 def extract_locale(path)

spec/integrity/js_constants_spec.rb

@@ -1,7 +1,5 @@
 # frozen_string_literal: true
 
-require "rails_helper"
-
 describe "constants match ruby" do
 
   let(:ctx) { MiniRacer::Context.new }

spec/integrity/oj_spec.rb

@@ -1,7 +1,5 @@
 # frozen_string_literal: true
 
-require "rails_helper"
-
 describe 'Oj' do
   it "is enabled" do
     classes = Set.new

spec/integrity/onceoff_integrity_spec.rb

@@ -1,7 +1,5 @@
 # frozen_string_literal: true
 
-require "rails_helper"
-
 describe ::Jobs::Onceoff do
   it "can run all once off jobs without errors" do
     # Load all once offs

spec/integrity/site_setting_spec.rb

@@ -1,6 +1,5 @@
 # frozen_string_literal: true
 
-require "rails_helper"
 require "i18n/duplicate_key_finder"
 
 describe "site setting integrity checks" do