Fixes #23932 - Fixes failing application job tests for some plugins

The tests were failing with:
Sequel::DatabaseError: PG::DuplicateTable:
ERROR:  relation "dynflow_execution_plans" already exists

This failure was probably caused by forcing initialization of the
Dynflow world in a before test block.

test/unit/application_job_test.rb

@@ -3,13 +3,15 @@
 
 class ApplicationJobTest < ActiveSupport::TestCase
   describe '.spawn_if_missing' do
-    # Force world initialization before stubbing,
-    #   otherwise CreateRssNotifications would be triggered
-    #   on first call to world
-    before { world }
-
     let(:job_class) { ApplicationJob }
-    let(:world) { Foreman::Application.dynflow.world }
+
+    # Using real world led to various issues, let's stub it out
+    let(:world) do
+      persistence = mock()
+      persistence.stubs(:find_execution_plans).returns([])
+      persistence.stubs(:load_delayed_plan)
+      OpenStruct.new(:persistence => persistence)
+    end
 
     def stub_delayed_plans_with_serialized_args(*args)
       execution_plans = args.each_with_index.map { |_, index| OpenStruct.new(:id => index) }