Extend prerequisites for the "xfer" system test
The Net::DNS Perl module needs the Digest::HMAC module to support TSIG. However, since the latter is not a hard requirement for the former, some packagers do not make Net::DNS depend on Digest::HMAC. If Net::DNS is installed on a host but Digest::HMAC is not, the "xfer" system test breaks in a very hard-to-debug way (ans5 returns TSIG RRs with empty RDATA, which prevents TSIG-signed SOA queries and transfers from working). Prevent this from happening by making the "xfer" system test explicitly require Digest::HMAC apart from Net::DNS.