Extend reimbursement feature spec

Reimbursements only work if there is a user with the email address
'spree@example.com', or if you explicitly pass the creator when creating
a reimbursement.

This commit adds to the testing set-up for this spec by mocking
a signed in admin creating the reimbursement, who can be passed as the
creator.

backend/spec/features/admin/orders/return_payment_state_spec.rb

@@ -7,9 +7,12 @@
 
   before do
     Spree::RefundReason.create!(name: Spree::RefundReason::RETURN_PROCESSING_REASON, mutable: false)
+    allow_any_instance_of(Spree::Admin::ReimbursementsController).to receive(:try_spree_current_user).
+      and_return(user)
   end
 
   let!(:order) { create(:shipped_order) }
+  let(:user) { create(:admin_user) }
 
   # Regression test for https://github.com/spree/spree/issues/6229
   it "refunds and has outstanding_balance of zero", js: true do