doth system test: increase transfers-in/out limits

doth system test: increase transfers-in/out limits

Sometimes doth test could intermittently fail shortly after start due
to inability to complete a zone transfer in time. As it turned out, it
could happen due to transfers-in/out limits. Initially the defaults
were fine, but over time, especially when adding Strict/Mutual TLS, we
added more than 10 zones so it became possible to hit the limits.

This commit takes care of that by bumping the limits.

(cherry picked from commit 95a551de7b91bbb10bab960860f45ed5a3e816d4)