Fixes clown faction runtimes. (tgstation#67707)

code/modules/mob/living/simple_animal/hostile/retaliate/clown.dm

@@ -35,7 +35,7 @@
 	unsuitable_atmos_damage = 10
 	unsuitable_heat_damage = 15
 	footstep_type = FOOTSTEP_MOB_SHOE
-	faction = FACTION_CLOWN
+	faction = list(FACTION_CLOWN)
 	var/attack_reagent
 
 /mob/living/simple_animal/hostile/retaliate/clown/Initialize(mapload)

code/modules/unit_tests/_unit_tests.dm

@@ -108,6 +108,7 @@
 #include "medical_wounds.dm"
 #include "merge_type.dm"
 #include "metabolizing.dm"
+#include "mob_faction.dm"
 #include "mob_spawn.dm"
 #include "modsuit.dm"
 #include "modular_map_loader.dm"

code/modules/unit_tests/mob_faction.dm

@@ -0,0 +1,19 @@
+/// Checks if any mob's faction var initial value is not a list, which is not supported by the current code
+/datum/unit_test/mob_faction
+
+/datum/unit_test/mob_faction/Run()
+	/// Right now taken from create_and_destroy
+	var/list/ignored = list(
+		/mob/living/carbon,
+		/mob/dview,
+		/mob/oranges_ear
+	)
+	ignored += typesof(/mob/camera/imaginary_friend)
+	ignored += typesof(/mob/living/simple_animal/pet/gondola/gondolapod)
+	ignored += typesof(/mob/living/silicon/robot/model)
+	ignored += typesof(/mob/camera/ai_eye/remote/base_construction)
+	ignored += typesof(/mob/camera/ai_eye/remote/shuttle_docker)
+	for (var/mob_type in typesof(/mob) - ignored)
+		var/mob/mob_instance = allocate(mob_type)
+		if(!islist(mob_instance.faction))
+			TEST_FAIL("[mob_type] faction variable is not a list")