Merge type variable fixes and unit test (#55173)

This PR fixes a case where certain materials caused issues when working with stacking machines, because they did not have set merge_type from the get go, which meant that initial() of that variable returned null.

To clarify further - if /obj/item/stack does not have set merge_type, it is generated merge_type upon Initialize(), which is the same as its typepath. For example, currently /obj/item/stack/sheet/bluespace_crystal does not have any merge_type set, and it is given merge_type = /obj/item/stack/sheet/bluespace_crystal upon Initialize(). Each Initialize(). Again and again.

There are quite a bit of these cases in the codebase, especially if its some older code. I have gone through them and set all of them their set merge_type, which they would inevitably receive anyway upon initializing and it fixes a bug mentioned above.

To prevent this happening again, I have also included unit test to check if merge types are set for stacks, included exceptions are usually abstract paths like /obj/item/stack/sheet/mineral, which contains zero behavior on its own and does not spawn unless done via admin tools.

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!")