Pick createEmptyInstance method from the origin repo

Author: theEvilReaper

src/main/java/net/minestom/testing/Env.java

@@ -149,6 +149,15 @@ default boolean tickWhile(@NotNull BooleanSupplier condition, @Nullable Duration
         return instance;
     }
 
+    /**
+     * Creates a new {@link Instance} which is empty and can be used in the test environment.
+     *
+     * @return the created instance
+     */
+    default @NotNull Instance createEmptyInstance() {
+        return process().instance().createInstanceContainer();
+    }
+
     /**
      * Destroys the given {@link Instance} from the test environment.
      * Note: This method does not remove players from the instance.