test-cmd.sh can fail if public IP not available

test-cmd runs a 127.0.0.1 version of the apiservers, but does not
set --public_address_override, which means it can fail on some systems
(like Macs) which don't expose a bindable external port by default.

Since reachability is not necessary for test-cmd.sh today, set
public_address_override to 127.0.0.1