Enable dig retries in legacy system test
This change partially reverts 47b85034,
which made +tries=1 +time=2 the default dig options to speed up dig
timeouts because several timeouts are expected in the legacy test.
Test runtime was more than halved as a result. However, defaulting to
+tries=1 prevents dig from retrying, which is not helpful when the
system is under a high load. We can keep the decreased test runtime
while letting most dig instances retry if we disable retry only when dig
is expected to fail.
Closes #3916 (closed)