Add TNT test structure

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

The structure was added to reduce manual work when examining or
verifying TNT explosion results after changes to TNT drop rate.

mods/MAPGEN/mcl_structures/init.lua

@@ -542,9 +542,14 @@ mcl_structures.generate_test_structure_fireproof = function(pos, rotation, pr)
 	mcl_structures.place_schematic(pos, path, rotation, nil, true, nil, nil, pr)
 end
 
+mcl_structures.generate_test_structure_tnt = function(pos, rotation, pr)
+	local path = minetest.get_modpath("mcl_structures").."/schematics/mcl_structures_test_structure_tnt.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 | test_structure_comparator | test_structure_fireproof",
+	params = "desert_temple | desert_well | igloo | witch_hut | boulder | ice_spike_small | ice_spike_large | fossil | end_exit_portal | end_portal_shrine | test_structure_comparator | test_structure_fireproof | test_structure_tnt",
 	description = S("Generate a pre-defined structure near your position."),
 	privs = {debug = true},
 	func = function(name, param)
@@ -582,6 +587,8 @@ minetest.register_chatcommand("spawnstruct", {
 			mcl_structures.generate_test_structure_comparator(pos, rot, pr)
 		elseif param == "test_structure_fireproof" then
 			mcl_structures.generate_test_structure_fireproof(pos, rot, pr)
+		elseif param == "test_structure_tnt" then
+			mcl_structures.generate_test_structure_tnt(pos, rot, pr)
 		elseif param == "" then
 			message = S("Error: No structure type given. Please use “/spawnstruct <type>”.")
 			errord = true