Add regression test against habtm memoized singular_ids

Starting in Rails 5.0.0 and still present in Rails 5.2.1, `singular_ids`
got memoized and didn't reload after more items were added to the
relation.

Although 19c8071 happens to fix the issue, it only adds tests for
`has_many` relations while this bug only affected
`has_and_belongs_to_many` relations.

This commit adds a regression test to ensure it never happens again with
`habtm` relations.

Ensures #34179 never gets reproduced.

activerecord/test/cases/associations/has_and_belongs_to_many_associations_test.rb

@@ -25,6 +25,8 @@ require "models/user"
 require "models/member"
 require "models/membership"
 require "models/sponsor"
+require "models/lesson"
+require "models/student"
 require "models/country"
 require "models/treaty"
 require "models/vertex"
@@ -780,6 +782,16 @@ class HasAndBelongsToManyAssociationsTest < ActiveRecord::TestCase
     assert_equal [projects(:active_record), projects(:action_controller)].map(&:id).sort, developer.project_ids.sort
   end
 
+  def test_singular_ids_are_reloaded_after_collection_concat
+    student = Student.create(name: "Alberto Almagro")
+    student.lesson_ids
+
+    lesson = Lesson.create(name: "DSI")
+    student.lessons << lesson
+
+    assert_includes student.lesson_ids, lesson.id
+  end
+
   def test_scoped_find_on_through_association_doesnt_return_read_only_records
     tag = Post.find(1).tags.find_by_name("General")