Give test grenade a proper name

Author: rm-code

res/data/items/weapons/test_grenade.lua renamed to res/data/items/weapons/m67_grenade.lua

@@ -1,5 +1,5 @@
 return {
-    name     = "Test Grenade",
+    name     = "M67 Grenade",
     itemType = "Weapon",
     weaponType = "Grenade",
     damage   = 120,