Make `maintain_test_schema` a module instance variable

Followup: https://github.com/rails/rails/pull/42442

activerecord/lib/active_record.rb

@@ -200,6 +200,9 @@ module ActiveRecord
   singleton_class.attr_accessor :queues
   self.queues = {}
 
+  singleton_class.attr_accessor :maintain_test_schema
+  self.maintain_test_schema = nil
+
   ##
   # :singleton-method:
   # Specify a threshold for the size of query result sets. If the number of

activerecord/lib/active_record/core.rb

@@ -65,8 +65,6 @@ module ActiveRecord
       # to the database while the app is running.
       class_attribute :enumerate_columns_in_select_statements, instance_accessor: false, default: false
 
-      mattr_accessor :maintain_test_schema, instance_accessor: false
-
       class_attribute :belongs_to_required_by_default, instance_accessor: false
 
       class_attribute :strict_loading_by_default, instance_accessor: false, default: false

activerecord/lib/active_record/migration.rb

@@ -651,7 +651,7 @@ module ActiveRecord
       end
 
       def maintain_test_schema! #:nodoc:
-        if ActiveRecord::Base.maintain_test_schema
+        if ActiveRecord.maintain_test_schema
           suppress_messages { load_schema_if_pending! }
         end
       end