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/lib/site_settings/db_provider_spec.rb

@@ -1,7 +1,5 @@
 # frozen_string_literal: true
 
-require 'rails_helper'
-
 describe SiteSettings::DbProvider do
 
   def expect_same_setting(actual, expected)

spec/lib/site_settings/defaults_provider_spec.rb

@@ -1,7 +1,5 @@
 # frozen_string_literal: true
 
-require 'rails_helper'
-
 describe SiteSettings::DefaultsProvider do
   let(:provider_local) do
     SiteSettings::LocalProcessProvider.new

spec/lib/site_settings/local_process_provider_multisite_spec.rb

@@ -1,7 +1,5 @@
 # frozen_string_literal: true
 
-require "rails_helper"
-
 describe SiteSettings::LocalProcessProvider, type: :multisite do
   def expect_same_setting(actual, expected)
     expect(actual.name).to eq(expected.name)

spec/lib/site_settings/local_process_provider_spec.rb

@@ -1,7 +1,5 @@
 # frozen_string_literal: true
 
-require 'rails_helper'
-
 describe SiteSettings::LocalProcessProvider do
 
   def expect_same_setting(actual, expected)

spec/lib/site_settings/type_supervisor_spec.rb

@@ -1,7 +1,5 @@
 # frozen_string_literal: true
 
-require 'rails_helper'
-
 describe SiteSettings::TypeSupervisor do
   let :provider_local do
     SiteSettings::LocalProcessProvider.new

spec/lib/site_settings/validations_spec.rb

@@ -1,6 +1,5 @@
 # frozen_string_literal: true
 
-require 'rails_helper'
 require 'site_settings/validations'
 
 describe SiteSettings::Validations do

spec/lib/site_settings/yaml_loader_spec.rb

@@ -1,6 +1,5 @@
 # frozen_string_literal: true
 
-require 'rails_helper'
 require 'site_settings/yaml_loader'
 
 describe SiteSettings::YamlLoader do