Update producer mailer specs to check that this producer facing email remains unaffected by white labelling (there are no customer facing emails here)

Also make use of the newly separated shared_examples

spec/mailers/producer_mailer_spec.rb

@@ -66,6 +66,11 @@ RSpec.describe ProducerMailer do
     expect(mail.reply_to).to eq [order_cycle.coordinator.contact.email]
   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
+
   it "includes the pickup time for each distributor" do
     expect(mail.body.encoded).to include "#{d1.name} (Tue, 23rd Dec)"
   end