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 #8234 (closed)

CE port: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/22764