Hmm, just retried it twice more, and the test succeeds on these two tests. Perhaps Net::DNS gives up too quickly?
Found the section in "man Mail::SpamAssassin::Conf" on dns_available and changed it to use a domain my server is master for, to guarantee that it has the records on hand if it's up. In /etc/mail/spamassassin/local.cf I now include:
# DNS tests may time out prematurely indicating falsely that BIND is # down so query a zone we are authoritative for locally dns_available test: myhouse.lan
