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.