Update backorder mailer specs to check that these hub facing emails remain unaffected by white labelling (there are no customer facing emails here)

Also make use of the newly separated shared_examples

spec/mailers/backorder_mailer_spec.rb

@@ -6,18 +6,30 @@ RSpec.describe BackorderMailer do
   let(:order) { create(:completed_order_with_totals) }
 
   describe "#backorder_failed" do
+    subject(:mail) { described_class.backorder_failed(order) }
+
     it "notifies the owner" do
       order.distributor.owner.email = "jane@example.net"
 
       BackorderMailer.backorder_failed(order).deliver_now
 
-      mail = ActionMailer::Base.deliveries.first
-      expect(mail.to).to eq ["jane@example.net"]
-      expect(mail.subject).to eq "An automatic backorder failed"
+      first_mail = ActionMailer::Base.deliveries.first
+      expect(first_mail.to).to eq ["jane@example.net"]
+      expect(first_mail.subject).to eq "An automatic backorder failed"
+    end
+
+    context "white labelling" do
+      it_behaves_like 'email with inactive white labelling', :mail
+      it_behaves_like 'non-customer facing email with active white labelling', :mail
     end
   end
 
   describe "#backorder_incomplete" do
+    subject(:mail) {
+      described_class.backorder_incomplete(
+        user, distributor, order_cycle, order_id
+      )
+    }
     let(:user) { build(:user, email: "jane@example.net") }
     let(:distributor) { build(:enterprise) }
     let(:order_cycle) { build(:order_cycle) }
@@ -26,9 +38,14 @@ RSpec.describe BackorderMailer do
     it "notifies the owner" do
       BackorderMailer.backorder_incomplete(user, distributor, order_cycle, order_id).deliver_now
 
-      mail = ActionMailer::Base.deliveries.first
-      expect(mail.to).to eq ["jane@example.net"]
-      expect(mail.subject).to eq "An automatic backorder failed to complete"
+      first_mail = ActionMailer::Base.deliveries.first
+      expect(first_mail.to).to eq ["jane@example.net"]
+      expect(first_mail.subject).to eq "An automatic backorder failed to complete"
+    end
+
+    context "white labelling" do
+      it_behaves_like 'email with inactive white labelling', :mail
+      it_behaves_like 'non-customer facing email with active white labelling', :mail
     end
   end
 end