Fix resolver tests: prefetch 40/41

Fix resolver tests: prefetch 40/41

These two tests were failing basically because in order for prefetching to
happen, the TTL for a given DNS record must be greater than or equal to
the prefetch config value + 9.

The previous TTL for both records was 10, while prefetch value in
configuration was 3, thus making only records with TTL >= 12 elligible
for prefetching.

TTL value for both records was adjusted to the value 13, and prefetch
value was set to 4 (inc by 1), so records with TTL (4 + 9) >= 13 are
elligible for prefetching.

Adjusting prefetch value to 4 gives the test 1 second more to avoid time
problems when sharing resources on a heavy loaded PC.

Also prefetch value in settings is now read by the script and used
by it to corrrectly calculate the amount of time needed to delay before
sending a request to trigger prefetch, adding a bit of flexibility to
fine tune the test in the future.

(cherry picked from commit a711d6f8c012d65da2635e857e0d6a04b859c428)