Fix command computers being exposed as peripherals

- Check whether the computer is a command computer before registering
   the capability.

 - Add tests to check what is/isn't a peripheral. See also #2020, where
   we forgot to register a peripheral on NeoForge 1.21.1.

Fixes #2070.

projects/forge/src/main/java/dan200/computercraft/shared/ForgeCommonHooks.java

@@ -124,7 +124,7 @@ public class ForgeCommonHooks {
     @SubscribeEvent
     public static void onCapability(AttachCapabilitiesEvent<BlockEntity> event) {
         var blockEntity = event.getObject();
-        if (blockEntity instanceof ComputerBlockEntity computer) {
+        if (blockEntity instanceof ComputerBlockEntity computer && !computer.isAdminOnly()) {
             CapabilityProvider.attach(event, PERIPHERAL, CAPABILITY_PERIPHERAL, computer::peripheral);
         } else if (blockEntity instanceof TurtleBlockEntity turtle) {
             CapabilityProvider.attach(event, INVENTORY, ITEM_HANDLER, () -> new InvWrapper(turtle));