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/onebox/engine/html_spec.rb

@@ -1,7 +1,5 @@
 # frozen_string_literal: true
 
-require "rails_helper"
-
 describe Onebox::Engine::HTML do
   before do
     @link = "http://amazon.com"