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/imap/providers/detector_spec.rb

@@ -1,7 +1,5 @@
 # frozen_string_literal: true
 
-require 'rails_helper'
-
 RSpec.describe Imap::Providers::Detector do
   it "returns the gmail provider if the gmail imap server is used" do
     config = {

spec/lib/imap/providers/generic_spec.rb

@@ -1,7 +1,5 @@
 # frozen_string_literal: true
 
-require 'rails_helper'
-
 RSpec.describe Imap::Providers::Generic do
   fab!(:username) { "test@generic.com" }
   fab!(:password) { "test1!" }

spec/lib/imap/providers/gmail_spec.rb

@@ -1,8 +1,6 @@
 
 # frozen_string_literal: true
 
-require 'rails_helper'
-
 RSpec.describe Imap::Providers::Gmail do
   fab!(:username) { "test@generic.com" }
   fab!(:password) { "test1!" }

spec/lib/imap/sync_spec.rb

@@ -1,6 +1,5 @@
 # frozen_string_literal: true
 
-require 'rails_helper'
 require 'imap/sync'
 
 describe Imap::Sync do