Fixes topdown emissives/emissive blockers and blood trail rendering (#92430)

## About The Pull Request

Fixes this

<img width="983" height="591" alt="zen_Mhqh2OqiU4"
src="https://github.com/user-attachments/assets/b3275052-1b24-404b-82bc-a4c8e88bdbcf"
/>

This code is mildly bad, but this is the best way we can fix FLOAT_LAYER
topdown emissives/blockers rendering ontop of everything else.

Also added logging/unit testing for blood trails spawned outside of
holders, mappers can use holders to spawn trails (which is the right way
to add them to your maps)

## Changelog
🆑
fix: Fixed blood/glass floor glow going over objects
/🆑

code/modules/unit_tests/unit_test.dm

@@ -276,8 +276,10 @@ GLOBAL_VAR_INIT(focused_tests, focused_tests())
 		/obj/effect/bug_moving,
 		//The abstract grown item expects a seed, but doesn't have one
 		/obj/item/food/grown,
-		///Single use case holder atom requiring a user
+		//Single use case holder atom requiring a user
 		/atom/movable/looking_holder,
+		//Should not exist outside of holders
+		/obj/effect/decal/cleanable/blood/trail,
 	)
 
 	// Everything that follows is a typesof() check.