reduce timing dependencies in system tests

reduce timing dependencies in system tests

one of the tests in the resolver system test depends on dig
getting no response to its first two query attempts, and SERVFAIL
on the third after resolution times out.

using a 5-second retry timer in dig means the SERVFAIL response
could occur while dig is discarding the second query and preparing
to send the third. in this case the server's response could be
missed.  shortening the retry interval to 4 seconds ensures that
dig has already sent the third query when the SERVFAIL response
arrives.

also, the serve-stale system test could fail due to a race in which
it timed out after waiting ten seconds for a file to be written, and
the dig timeout was just a bit longer. this is addressed by extending
the dig timeout to 11 seconds for this test.