16470: Adds ArvadosModel test exposing a bug with audit logs.

16470: Adds ArvadosModel test exposing a bug with audit logs.

When audit logs are enabled, fetching objects from models with serialized
attributes (for example: User or ContainerRequest) return an unpersisted
instance even if reload() is called on it.
This is a problem because from Rails 5.2, lock!() and with_lock() will raise
an exception when called on unpersisted instances.

Arvados-DCO-1.1-Signed-off-by: Lucas Di Pentima <lucas@di-pentima.com.ar>