Fixes an issue with nightmare revival, Unit tests some fully heal stuff (#73612)

## About The Pull Request

- Same issue as Ethereals. Owner was `null`ed because the heart was
recreated. I opted for a more permanent solution, that being introducing
a new flag to avoid recreating organs.

- Adds some unit tests for fully heal stuff to make sure it works. 

## Why It's Good For The Game

More cases of revival working as expected

## Changelog

🆑 Melbert
fix: Nightmare revival acts less funky - stops it from re-creating the
Light Eater.
/🆑

---------

Co-authored-by: san7890 <the@san7890.com>

code/modules/unit_tests/_unit_tests.dm

@@ -114,6 +114,7 @@
 #include "emoting.dm"
 #include "focus_only_tests.dm"
 #include "food_edibility_check.dm"
+#include "full_heal.dm"
 #include "gas_transfer.dm"
 #include "get_turf_pixel.dm"
 #include "geyser.dm"

code/modules/unit_tests/full_heal.dm

@@ -0,0 +1,60 @@
+/// Tests the fully heal flag [HEAL_ORGANS].
+/datum/unit_test/full_heal_heals_organs
+
+/datum/unit_test/full_heal_heals_organs/Run()
+	var/mob/living/carbon/human/dummy = allocate(/mob/living/carbon/human/consistent)
+
+	for(var/obj/item/organ/internal/organ in dummy.internal_organs)
+		organ.applyOrganDamage(50)
+
+	dummy.fully_heal(HEAL_ORGANS)
+
+	for(var/obj/item/organ/internal/organ in dummy.internal_organs)
+		if(organ.damage <= 0)
+			continue
+		TEST_FAIL("Organ [organ] did not get healed by fullyheal flag HEAL_ORGANS.")
+
+/// Tests the fully heal flag [HEAL_REFRESH_ORGANS].
+/datum/unit_test/full_heal_regenerates_organs
+
+/datum/unit_test/full_heal_regenerates_organs/Run()
+	var/mob/living/carbon/human/dummy = allocate(/mob/living/carbon/human/consistent)
+
+	var/list/we_started_with = list()
+
+	for(var/obj/item/organ/internal/organ in dummy.internal_organs)
+		if(organ.organ_flags & ORGAN_VITAL) // leave this for now
+			continue
+		we_started_with += organ.type
+		qdel(organ)
+
+	TEST_ASSERT(length(we_started_with), "Dummy didn't spawn with any organs to regenerate.")
+
+	dummy.fully_heal(HEAL_REFRESH_ORGANS)
+
+	for(var/obj/item/organ/organ_type as anything in we_started_with)
+		if(dummy.getorgan(organ_type))
+			continue
+		TEST_FAIL("Organ [initial(organ_type.name)] didn't regenerate in the dummy after fullyheal flag HEAL_REFRESH_ORGANS.")
+
+/// Tests the fully heal combination flag [HEAL_DAMAGE].
+/datum/unit_test/full_heal_damage_types
+
+/datum/unit_test/full_heal_damage_types/Run()
+	var/mob/living/carbon/human/dummy = allocate(/mob/living/carbon/human/consistent)
+
+	dummy.apply_damages(brute = 10, burn = 10, tox = 10, oxy = 10, clone = 10, stamina = 10)
+	dummy.fully_heal(HEAL_DAMAGE)
+
+	if(dummy.getBruteLoss())
+		TEST_FAIL("The dummy still had brute damage after a fully heal!")
+	if(dummy.getFireLoss())
+		TEST_FAIL("The dummy still had burn damage after a fully heal!")
+	if(dummy.getToxLoss())
+		TEST_FAIL("The dummy still had toxins damage after a fully heal!")
+	if(dummy.getOxyLoss())
+		TEST_FAIL("The dummy still had oxy damage after a fully heal!")
+	if(dummy.getCloneLoss())
+		TEST_FAIL("The dummy still had clone damage after a fully heal!")
+	if(dummy.getStaminaLoss())
+		TEST_FAIL("The dummy still had stamina damage after a fully heal!")