/test pos is offset by one on the y and z axis

The in-world render as well as the chat messages report an incorrect position when using the /test pos command. That reported position seems to be relative to the test instance block, instead of the structure origin.

Steps to reproduce:
1. Place a test instance block
2. Set the test structure size to 1 1 1 and press done
3. Place a block within the structure
4. Look at the block and run /test pos

Observed behavior:
The reported position reads 0, 1, 1

Expected behavior:
The reported position should read 0, 0, 0, as the checked block is at the structure origin.