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/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