Add (theoretically) fireproof test structure

This patch adds a new test structure to the “/spawnstruct” command which
can be spawned with this command: /spawnstruct test_structure_fireproof

The structure can be used to verify that eternal fire can be spawned by
the structure placement code. It can also be used to debug fire spread,
as according to Minecraft fire spread rules, it should be fireproof.

See <https://minecraft.fandom.com/wiki/Fire#Spread> for those rules.

mods/MAPGEN/mcl_structures/init.lua

@@ -532,9 +532,14 @@ local function dir_to_rotation(dir)
 	return "0"
 end
 
+mcl_structures.generate_test_structure_fireproof = function(pos, rotation, pr)
+	local path = minetest.get_modpath("mcl_structures").."/schematics/mcl_structures_test_structure_fireproof.mts"
+	mcl_structures.place_schematic(pos, path, rotation, nil, true, nil, nil, pr)
+end
+
 -- Debug command
 minetest.register_chatcommand("spawnstruct", {
-	params = "desert_temple | desert_well | igloo | witch_hut | boulder | ice_spike_small | ice_spike_large | fossil | end_exit_portal | end_portal_shrine",
+	params = "desert_temple | desert_well | igloo | witch_hut | boulder | ice_spike_small | ice_spike_large | fossil | end_exit_portal | end_portal_shrine | test_structure_fireproof",
 	description = S("Generate a pre-defined structure near your position."),
 	privs = {debug = true},
 	func = function(name, param)
@@ -568,6 +573,8 @@ minetest.register_chatcommand("spawnstruct", {
 			mcl_structures.generate_end_exit_portal(pos, rot, pr)
 		elseif param == "end_portal_shrine" then
 			mcl_structures.generate_end_portal_shrine(pos, rot, pr)
+		elseif param == "test_structure_fireproof" then
+			mcl_structures.generate_test_structure_fireproof(pos, rot, pr)
 		elseif param == "" then
 			message = S("Error: No structure type given. Please use “/spawnstruct <type>”.")
 			errord = true