create_and_destroy will no longer spawn abstract types (#95071)

## About The Pull Request

What it says on the tin-- with having a nice abstract types system now,
we can utilize that in create_and_destroy.

## Why It's Good For The Game

Removes a lot of the need for snowflake item exclusions, and makes this
test likely a lot more stable (and a little faster even).

## Changelog

Not player-facing

code/modules/mob_spawn/mob_spawn.dm

@@ -5,6 +5,7 @@
 	//So it shows up in the map editor
 	icon = 'icons/effects/mapping_helpers.dmi'
 	icon_state = "mobspawner"
+	abstract_type = /obj/effect/mob_spawn
 	/// Can this spawner be used up?
 	var/infinite_use = FALSE
 	///A forced name of the mob, though can be overridden if a special name is passed as an argument