Ensure ActiveRecord::Base has descendants before checking triggers (#14732)

HairTrigger uses the ActiveRecord::Base.descendants to find triggers
in the db (from `models`). WithModel (used in the Settings::Base spec)
clears this during its descendants cleanup, causing this test to
ocassionally fail.

Before checking hairtrigger, if AR::Base descendents is empty, add
ApplicationRecord (there's an assumption here that all of our
application models inherit from application record - this may or may
not be true if we use engine generated models that do not follow this
convention and need to validate triggers there.

spec/models/hair_trigger_spec.rb

@@ -5,6 +5,12 @@ require "rails_helper"
 RSpec.describe HairTrigger, type: :model do
   describe ".migrations_current?" do
     it "is always true" do
+      # work-around empty AR::Base descendants array caused by with_model cleanup
+      # HairTrigger uses AR::Base to get database triggers (and compare against the schema)
+      if ActiveRecord::Base.descendants.blank?
+        ActiveSupport::DescendantsTracker.store_inherited(ActiveRecord::Base, ApplicationRecord)
+      end
+
       expect(described_class.migrations_current?).to be(true)
     end
   end