Enable spec files to be executed alone

This fixes the annoying error

```
NameError:
  uninitialized constant OrderManagement::Subscriptions::Whatever
```

and let's you execute the spec file in isolation. It slows down way too
much having the run the entire engine test suite while developing. And
it makes me nervous too.

engines/order_management/spec/services/order_management/subscriptions/count_spec.rb

@@ -1,5 +1,7 @@
 # frozen_string_literal: true
 
+require 'spec_helper'
+
 module OrderManagement
   module Subscriptions
     describe Count do

engines/order_management/spec/services/order_management/subscriptions/estimator_spec.rb

@@ -1,5 +1,7 @@
 # frozen_string_literal: true
 
+require 'spec_helper'
+
 module OrderManagement
   module Subscriptions
     describe Estimator do

engines/order_management/spec/services/order_management/subscriptions/proxy_order_syncer_spec.rb

@@ -1,5 +1,7 @@
 # frozen_string_literal: true
 
+require 'spec_helper'
+
 module OrderManagement
   module Subscriptions
     describe ProxyOrderSyncer do

engines/order_management/spec/services/order_management/subscriptions/summary_spec.rb

@@ -1,5 +1,7 @@
 # frozen_string_literal: true
 
+require 'spec_helper'
+
 module OrderManagement
   module Subscriptions
     describe Summary do