Prevent attr_encrypted models from being overriden

Fix failing spec in spec/controllers/admin/hooks_controller_spec.rb

attr_encrypted expects models to have their attribute methods defined,
or it will override them with standard Ruby accessors. Migration specs
that rolled back the state of the database after columns were migrated
to encrypted values were interfering with these definitions. To ensure
that the SystemHook specs pass, we need to call
`SystemHook.define_attribute_methods` to ensure that attr_encrypted sees
the right methods that reflect the latest state of the database.

Closes https://gitlab.com/gitlab-org/gitlab-ee/issues/8234

spec/support/helpers/migrations_helpers.rb

 module MigrationsHelpers
+  def active_record_base
+    ActiveRecord::Base
+  end
+
   def table(name)
-    Class.new(ActiveRecord::Base) do
+    Class.new(active_record_base) do
       self.table_name = name
       self.inheritance_column = :_type_disabled
 
@@ -19,7 +23,7 @@ module MigrationsHelpers
   end
 
   def clear_schema_cache!
-    ActiveRecord::Base.connection_pool.connections.each do |conn|
+    active_record_base.connection_pool.connections.each do |conn|
       conn.schema_cache.clear!
     end
   end
@@ -40,11 +44,18 @@ module MigrationsHelpers
     # Reset column information for the most offending classes **after** we
     # migrated the schema up, otherwise, column information could be
     # outdated. We have a separate method for this so we can override it in EE.
-    ActiveRecord::Base.descendants.each(&method(:reset_column_information))
+    active_record_base.descendants.each(&method(:reset_column_information))
+  end
 
-    # Without that, we get errors because of missing attributes, e.g.
+  def refresh_attribute_methods
+    # Without this, we get errors because of missing attributes, e.g.
     # super: no superclass method `elasticsearch_indexing' for #<ApplicationSetting:0x00007f85628508d8>
-    ApplicationSetting.define_attribute_methods
+    # attr_encrypted also expects ActiveRecord attribute methods to be
+    # defined, or it will override the accessors:
+    # https://gitlab.com/gitlab-org/gitlab-ee/issues/8234#note_113976421
+    [ApplicationSetting, SystemHook].each do |model|
+      model.define_attribute_methods
+    end
   end
 
   def reset_column_information(klass)
@@ -84,6 +95,7 @@ module MigrationsHelpers
     end
 
     reset_column_in_all_models
+    refresh_attribute_methods
   end
 
   def disable_migrations_output