[9.20] 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 isc-projects/bind9#4943.

Backport of MR !9507

Merge branch 'backport-mnowak/shutdown-downgrade-timeout-and-attempts-arguments-9.20' into 'bind-9.20'

See merge request isc-projects/bind9!9541