Merge type variable fixes and unit test (#1867)

Co-authored-by: Arkatos1 <43862960+Arkatos1@users.noreply.github.com>
Co-authored-by: Gandalf <jzo123@hotmail.com>

code/modules/unit_tests/_unit_tests.dm

@@ -45,6 +45,7 @@
 #include "keybinding_init.dm"
 #include "machine_disassembly.dm"
 #include "medical_wounds.dm"
+#include "merge_type.dm"
 #include "metabolizing.dm"
 #include "outfit_sanity.dm"
 #include "pills.dm"

code/modules/unit_tests/merge_type.dm

@@ -0,0 +1,15 @@
+/datum/unit_test/merge_type/Run()
+	var/list/blacklist = list(/obj/item/stack/sheet,
+							  /obj/item/stack/sheet/mineral,
+							  /obj/item/stack/ore,
+							  /obj/item/stack/spacecash,
+							  /obj/item/stack/license_plates,
+							  /obj/item/stack/tile/mineral,
+							  /obj/item/stack/tile)
+
+	var/list/paths = subtypesof(/obj/item/stack) - blacklist
+
+	for(var/stackpath in paths)
+		var/obj/item/stack/stack = stackpath
+		if(!initial(stack.merge_type))
+			Fail("([stack]) lacks set merge_type variable!")