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/mining/ores_coins.dm

@@ -78,6 +78,7 @@
 	mine_experience = 6
 	scan_state = "rock_Uranium"
 	spreadChance = 5
+	merge_type = /obj/item/stack/ore/uranium
 
 /obj/item/stack/ore/iron
 	name = "iron ore"
@@ -90,6 +91,7 @@
 	mine_experience = 1
 	scan_state = "rock_Iron"
 	spreadChance = 20
+	merge_type = /obj/item/stack/ore/iron
 
 /obj/item/stack/ore/glass
 	name = "sand pile"
@@ -101,6 +103,7 @@
 	refined_type = /obj/item/stack/sheet/glass
 	w_class = WEIGHT_CLASS_TINY
 	mine_experience = 0 //its sand
+	merge_type = /obj/item/stack/ore/glass
 
 GLOBAL_LIST_INIT(sand_recipes, list(\
 		new /datum/stack_recipe("sandstone", /obj/item/stack/sheet/mineral/sandstone, 1, 1, 50),\
@@ -135,6 +138,7 @@ GLOBAL_LIST_INIT(sand_recipes, list(\
 	inhand_icon_state = "volcanic_sand"
 	singular_name = "volcanic ash pile"
 	mine_experience = 0
+	merge_type = /obj/item/stack/ore/glass/basalt
 
 /obj/item/stack/ore/plasma
 	name = "plasma ore"
@@ -147,12 +151,12 @@ GLOBAL_LIST_INIT(sand_recipes, list(\
 	mine_experience = 5
 	scan_state = "rock_Plasma"
 	spreadChance = 8
+	merge_type = /obj/item/stack/ore/plasma
 
 /obj/item/stack/ore/plasma/welder_act(mob/living/user, obj/item/I)
 	to_chat(user, "<span class='warning'>You can't hit a high enough temperature to smelt [src] properly!</span>")
 	return TRUE
 
-
 /obj/item/stack/ore/silver
 	name = "silver ore"
 	icon_state = "Silver ore"
@@ -164,6 +168,7 @@ GLOBAL_LIST_INIT(sand_recipes, list(\
 	refined_type = /obj/item/stack/sheet/mineral/silver
 	scan_state = "rock_Silver"
 	spreadChance = 5
+	merge_type = /obj/item/stack/ore/silver
 
 /obj/item/stack/ore/gold
 	name = "gold ore"
@@ -176,6 +181,7 @@ GLOBAL_LIST_INIT(sand_recipes, list(\
 	refined_type = /obj/item/stack/sheet/mineral/gold
 	scan_state = "rock_Gold"
 	spreadChance = 5
+	merge_type = /obj/item/stack/ore/gold
 
 /obj/item/stack/ore/diamond
 	name = "diamond ore"
@@ -187,6 +193,7 @@ GLOBAL_LIST_INIT(sand_recipes, list(\
 	refined_type = /obj/item/stack/sheet/mineral/diamond
 	mine_experience = 10
 	scan_state = "rock_Diamond"
+	merge_type = /obj/item/stack/ore/diamond
 
 /obj/item/stack/ore/bananium
 	name = "bananium ore"
@@ -198,6 +205,7 @@ GLOBAL_LIST_INIT(sand_recipes, list(\
 	refined_type = /obj/item/stack/sheet/mineral/bananium
 	mine_experience = 15
 	scan_state = "rock_Bananium"
+	merge_type = /obj/item/stack/ore/bananium
 
 /obj/item/stack/ore/titanium
 	name = "titanium ore"
@@ -210,6 +218,7 @@ GLOBAL_LIST_INIT(sand_recipes, list(\
 	mine_experience = 3
 	scan_state = "rock_Titanium"
 	spreadChance = 5
+	merge_type = /obj/item/stack/ore/titanium
 
 /obj/item/stack/ore/slag
 	name = "slag"
@@ -217,6 +226,7 @@ GLOBAL_LIST_INIT(sand_recipes, list(\
 	icon_state = "slag"
 	inhand_icon_state = "slag"
 	singular_name = "slag chunk"
+	merge_type = /obj/item/stack/ore/slag
 
 /obj/item/gibtonite
 	name = "gibtonite ore"