Add dummy /testfor command in Bukkit. Addresses BUKKIT-3813

This command only functions in command blocks so the bukkit command for it
simply spits out an error message.

By: Travis Watkins <amaranth@ubuntu.com>

paper-api/src/main/java/org/bukkit/command/SimpleCommandMap.java

@@ -55,6 +55,7 @@ public class SimpleCommandMap implements CommandMap {
         fallbackCommands.add(new ClearCommand());
         fallbackCommands.add(new GameRuleCommand());
         fallbackCommands.add(new EnchantCommand());
+        fallbackCommands.add(new TestForCommand());
     }
 
     public SimpleCommandMap(final Server server) {