Keep DIM1/DIM-1 subdirectories when cloning a vanilla world directory for Spigot (#4334)

tests/setuponlytests/vanilla_world_dir_for_spigot_server/docker-compose.yml

@@ -0,0 +1,15 @@
+services:
+  mc:
+    restart: "no"
+    image: ${IMAGE_TO_TEST:-itzg/minecraft-server}
+    environment:
+      EULA: "TRUE"
+      SETUP_ONLY: "TRUE"
+      TYPE: "PAPER"
+      WORLD: /worlds/world
+      # the following are only used to speed up test execution
+      VERSION: 1.18.1
+      PAPER_CUSTOM_JAR: /servers/fake.jar
+    volumes:
+      - ./worlds:/worlds:ro
+      - ./data:/data

tests/setuponlytests/vanilla_world_dir_for_spigot_server/verify.sh

@@ -0,0 +1,3 @@
+mc-image-helper assert fileExists world/level.dat && \
+mc-image-helper assert fileExists world_nether/DIM-1/some_nether_file && \
+mc-image-helper assert fileExists world_the_end/DIM1/some_end_file