test/config: verify that user/role is not deleted

When the configuration changes and the instance is reloaded with it,
some roles or users may have been removed from the config. In such case,
it would be destructive to delete/disable them on the instance, so
this test checks that all users and roles removed in config stay
on the instance and keep all the privileges.

Part of #8967

NO_DOC=test
NO_CHANGELOG=test