Only load AWS test credentials once

Whenever the tests run, the following error appeared:

    warning: already initialized constant AWSTestCredentials

This moves the AWSTestCredentials init from the individual specs to a
spec helper class: `spec/support/helpers/amazon_helpers.rb`.

Author: leesharma

spec/lib/amazon_product_api/item_lookup_endpoint_spec.rb

@@ -1,10 +1,9 @@
 # frozen_string_literal: true
 
 require 'amazon_product_api/item_lookup_endpoint'
+require 'support/helpers/amazon_helpers'
 
 describe AmazonProductAPI::ItemLookupEndpoint do
-  AWSTestCredentials = Struct.new(:access_key, :secret_key, :associate_tag)
-
   let(:aws_credentials) {
     AWSTestCredentials.new('aws_access_key',
                            'aws_secret_key',

spec/lib/amazon_product_api/item_search_endpoint_spec.rb

@@ -1,10 +1,9 @@
 # frozen_string_literal: true
 
 require 'amazon_product_api/item_search_endpoint'
+require 'support/helpers/amazon_helpers'
 
 describe AmazonProductAPI::ItemSearchEndpoint do
-  AWSTestCredentials = Struct.new(:access_key, :secret_key, :associate_tag)
-
   let(:aws_credentials) {
     AWSTestCredentials.new('aws_access_key',
                            'aws_secret_key',

spec/support/helpers/amazon_helpers.rb

@@ -0,0 +1,6 @@
+# frozen_string_literal: true
+
+# Mocks an AWS Credentials object
+AWSTestCredentials = Struct.new(:access_key,
+                                :secret_key,
+                                :associate_tag)