Mark most common flaky specs

This will trigger a CI when they fail

backend/spec/controllers/spree/admin/customer_returns_controller_spec.rb

@@ -118,7 +118,7 @@ module Admin
               expect(assigns(:rma_return_items)).to include(rma_return_item)
             end
 
-            it "loads the correct return authorization reasons" do
+            it "loads the correct return authorization reasons", :flaky do
               expect(assigns(:reasons)).to match_array([return_reason_1, return_reason_2, return_reason_3])
             end
           end

backend/spec/spec_helper.rb

@@ -36,6 +36,7 @@
 require 'spree/testing_support/authorization_helpers'
 require 'spree/testing_support/preferences'
 require 'spree/testing_support/controller_requests'
+require 'spree/testing_support/flaky'
 require 'spree/testing_support/flash'
 require 'spree/testing_support/url_helpers'
 require 'spree/testing_support/order_walkthrough'

core/spec/models/spree/order_inventory_spec.rb

@@ -171,7 +171,7 @@
         end
 
         context 'when there is not enough availability at any stock location' do
-          it 'falls-back selecting first non-shipped shipment that leaves from same stock_location' do
+          it 'falls-back selecting first non-shipped shipment that leaves from same stock_location', :flaky do
             shipment = subject.send(:determine_target_shipment, 1)
             shipment.reload
 

core/spec/spec_helper.rb

@@ -12,6 +12,7 @@
 
 require 'rspec/core'
 
+require 'spree/testing_support/flaky'
 require 'spree/testing_support/partial_double_verification'
 require 'spree/testing_support/silence_deprecations'
 require 'spree/testing_support/preferences'