Add a test case for unscoping `default_scope` in associations

Unscoping `default_scope` in associations has already supported (#17360
for preloading, c9cf8b8d for eager loading).

Fixes #20679.
Closes #16531.

activerecord/test/cases/associations/has_many_associations_test.rb

@@ -2371,8 +2371,9 @@ def test_collection_association_with_private_kernel_method
     car = Car.create!
     bulb = Bulb.create! name: "other", car: car
 
-    assert_equal bulb, Car.find(car.id).all_bulbs.first
-    assert_equal bulb, Car.includes(:all_bulbs).find(car.id).all_bulbs.first
+    assert_equal [bulb], Car.find(car.id).all_bulbs
+    assert_equal [bulb], Car.includes(:all_bulbs).find(car.id).all_bulbs
+    assert_equal [bulb], Car.eager_load(:all_bulbs).find(car.id).all_bulbs
   end
 
   test "raises RecordNotDestroyed when replaced child can't be destroyed" do