Make d/tests/validation less flaky

Avoid direct square-brackets tests since they wrongly end up as an exit code equal to one. Also, add a second external domain as fallback.

Currently, the validation test passes, but ends ups as FLAKY, since there is a check that makes the script returns 1. See for example this full autopkgtest log: https://ci.debian.net/data/autopkgtest/unstable/amd64/b/bind9/45274604/log.gz

 43s validation           FLAKY non-zero exit status 1

So the current results are misleading.