chg: test: Downgrade "timeout" and "attempts" arguments in shutdown

The shutdown system test sends queries when named is shutting down, not in an attempt to get answers but to destabilize the server into a crash. With isctest.query.udp() defaulting to try up to ten times with a ten-second timeout to get a response we don't care about from a likely terminated server, we make the test run much longer than needed because of retries and long timeouts.

Also, see #4943.