Removes require_dependency from the AR test suite

In the AR test suite require_dependency does not make much sense. Just
call vanilla require/load.

Note that in the test that made explicit use of it, there are no
autoload paths, and no constants have been autoloaded. In reality, the
code ended up calling Kernel#load.

activerecord/test/cases/multiple_db_test.rb

@@ -57,12 +57,11 @@ class MultipleDbTest < ActiveRecord::TestCase
     assert_equal e3.course.id, c2.id
   end
 
-  def test_course_connection_should_survive_dependency_reload
+  def test_course_connection_should_survive_reloads
     assert Course.connection
 
-    ActiveSupport::Dependencies.clear
-    Object.send(:remove_const, :Course)
-    require_dependency "models/course"
+    assert Object.send(:remove_const, :Course)
+    assert load("models/course.rb")
 
     assert Course.connection
   end

activerecord/test/models/college.rb

@@ -1,6 +1,6 @@
 # frozen_string_literal: true
 
-require_dependency "models/arunit2_model"
+require "models/arunit2_model"
 require "active_support/core_ext/object/with_options"
 
 class College < ARUnit2Model

activerecord/test/models/course.rb

@@ -1,6 +1,6 @@
 # frozen_string_literal: true
 
-require_dependency "models/arunit2_model"
+require "models/arunit2_model"
 
 class Course < ARUnit2Model
   belongs_to :college

activerecord/test/models/other_dog.rb

@@ -1,6 +1,6 @@
 # frozen_string_literal: true
 
-require_dependency "models/arunit2_model"
+require "models/arunit2_model"
 
 class OtherDog < ARUnit2Model
   self.table_name = "dogs"

activerecord/test/models/professor.rb

@@ -1,6 +1,6 @@
 # frozen_string_literal: true
 
-require_dependency "models/arunit2_model"
+require "models/arunit2_model"
 
 class Professor < ARUnit2Model
   has_and_belongs_to_many :courses