make dupsigs test less timing-sensitive

make dupsigs test less timing-sensitive

the dupsigs test is prone to failing on slow CI machines
because the first test can occur before the zone is fully
signed.

instead of just waiting ten seconds arbitrarily, we now
check every second, and allow up to 30 seconds before giving
up.

(cherry picked from commit d9b85cbaaeda81d5f5351a1b463ad7e509bf76cf)