Update test mailer specs to check that this super admin facing email remains unaffected by white labelling (there are no user or customer facing emails here)

Also make use of the newly separated shared_examples

spec/mailers/test_mailer_spec.rb

@@ -3,7 +3,9 @@
 require 'spec_helper'
 
 RSpec.describe Spree::TestMailer do
+  subject(:mail) { described_class.test_email(order) }
   let(:user) { create(:user) }
+  let(:order) { build(:order_with_distributor) }
 
   context ":from not set explicitly" do
     it "falls back to spree config" do
@@ -18,4 +20,9 @@ RSpec.describe Spree::TestMailer do
       Spree::TestMailer.test_email(user.id).deliver_now
     }.not_to raise_error
   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