Properly unreg's turf changed when space openspace is deleted (#76501)

## About The Pull Request

Openspace tracks the turf below for starlight purposes The trouble is if
a turf is replaced by another, all its signal registers still persist

I forgot about this, so we'd get runtimes when a
/turf/open/space/openspace was replaced, and the turf below it changed

## Why It's Good For The Game

Less random runtimes

code/modules/unit_tests/create_and_destroy.dm

@@ -101,6 +101,9 @@ GLOBAL_VAR_INIT(running_create_and_destroy, FALSE)
 	ignore += subtypesof(/obj/machinery/airlock_controller)
 	// Always ought to have an associated escape menu. Any references it could possibly hold would need one regardless.
 	ignore += subtypesof(/atom/movable/screen/escape_menu)
+	// Can't spawn openspace above nothing, it'll get pissy at me
+	ignore += typesof(/turf/open/space/openspace)
+	ignore += typesof(/turf/open/openspace)
 
 	var/list/cached_contents = spawn_at.contents.Copy()
 	var/original_turf_type = spawn_at.type

code/modules/unit_tests/focus_only_tests.dm

@@ -36,5 +36,8 @@
 /// Checks for bad icon / icon state setups in cooking crafting menu
 /datum/unit_test/focus_only/bad_cooking_crafting_icons
 
+/// Ensures openspace never spawns on the bottom of a z stack
+/datum/unit_test/focus_only/openspace_clear
+
 /// Checks to ensure that variables expected to exist in a job datum (for config reasons) actually exist
 /datum/unit_test/focus_only/missing_job_datum_variables