[9.18] chg: test: Mark test_idle_timeout as flaky on FreeBSD 13

The test_idle_timeout check in the "timeouts" system test has been
failing often on FreeBSD 13 AWS hosts.  Adding timestamped debug logging
shows that the time.sleep() calls used in that check are returning
significantly later than asked to on that platform (e.g. after 4 seconds
when just 1 second is requested), breaking the test's timing assumptions
and triggering false positives.  These failures are not an indication of
a bug in named and have not been observed on any other platform.  Mark
the problematic check as flaky, but only on FreeBSD 13, so that other
failure modes are caught appropriately.

Backport of MR !10459

Merge branch 'backport-michal/mark-test_idle_timeout-as-flaky-on-freebsd-13-9.18' into 'bind-9.18'

See merge request isc-projects/bind9!10462