Make spec/models/mail_handler_spec.rb faster

Using `shared_let` for caching project creation lead to almost twice faster spec run time (33 secs to 17 secs).

spec/models/mail_handler_spec.rb

@@ -29,18 +29,15 @@
 require 'spec_helper'
 
 RSpec.describe MailHandler do
-  let(:anno_user) { User.anonymous }
-  let(:project) { create(:valid_project, identifier: 'onlinestore', name: 'OnlineStore', public: false) }
-  let(:public_project) { create(:valid_project, identifier: 'onlinestore', name: 'OnlineStore', public: true) }
-  let(:priority_low) { create(:priority_low, is_default: true) }
+  # we need these run first so the anonymous and system users are created and
+  # there is a default work package priority to save any work packages
+  shared_let(:anno_user) { User.anonymous }
+  shared_let(:system_user) { User.system }
+  shared_let(:priority_low) { create(:priority_low, name: 'Low', is_default: true) }
 
-  before do
-    # we need both of these run first so the anonymous user is created and
-    # there is a default work package priority to save any work packages
-    priority_low
-    anno_user
-    User.system
+  shared_let(:project) { create(:valid_project, identifier: 'onlinestore', name: 'OnlineStore', public: false) }
 
+  before do
     allow(UserMailer)
       .to receive(:incoming_email_error)
       .and_return instance_double(ActionMailer::MessageDelivery, deliver_later: nil)
@@ -874,8 +871,9 @@
         end
 
         subject do
+          project.update(public: true)
           submit_email('ticket_from_emission_address.eml',
-                       issue: { project: public_project.identifier },
+                       issue: { project: project.identifier },
                        unknown_user: 'create')
         end
 
@@ -918,7 +916,6 @@
       context 'for wp with status case insensitive' do
         let(:type) { project.types.first }
         let!(:status) { create(:status, name: 'Resolved', workflow_for_type: type) }
-        let!(:priority_low) { create(:priority_low, name: 'Low', is_default: true) }
         let!(:version) { create(:version, name: 'alpha', project:) }
 
         # This email contains: 'Project: onlinestore' and 'Status: resolved'