check_retry_add_dns_command test doesn't actually retry

[tlhackque]

As noted in #691:

test 18 (check_retry_add_dns_command) is expected to exit with failure status, which generates a fair bit of output due to the test harness.

More interesting is that the retry uses a failing add_dns_* command, which doesn't really test much.

It would be better for the test to switch to a command that is expected work after the first failure, which would verify that the retry mechanism actually works.

[timkimber]

The retry test intentionally just checks that the dns_add script is called again if the DNS isn't updated, I'll look at adding logic to only fail the first time or make the tests less noisy