The command runs, and it provides a link status. But no "cable-pairs" output, which is what I presume you're looking for.... in which case you'll need to wait for an official reply from MikroTik support team. It works on my hAP ac though, so the function is definitely implemented in ROSv7. Hopefully just a driver issue of sorts
[admin@router1] > /interface/ethernet/cable-test numbers=2
name: ether3
status: link-ok
-- [Q quit|D dump|C-z pause]