Stop using custom method buscar_suscripcion to test side effects
Suscribible and Suscriptor both now use Suscripcion.suscribir in their #suscribe_a! method
Removed tests for #suscribe_a! and #suscribeme_a! as they are now trivial proxies to Suscripcion.suscribir
Suscripcion.suscribir now returns the existing suscripcion when trying to create a duplicated one