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.

bin/tests/system/xfer/prereq.sh

@@ -25,3 +25,9 @@ else
     echo_i "This test requires the Net::DNS library." >&2
     exit 1
 fi
+
+if ! $PERL -e 'use Digest::HMAC;' 2>/dev/null
+then
+    echo_i "This test requires the Digest::HMAC Perl module." >&2
+    exit 1
+fi