Add test case for different digest type

Change one of the test cases to use a different digest type (4).  The
system tests and kasp script need to be updated to take into account
the new algorithm (instead of the hard coded 2).

bin/tests/system/nsec3/tests.sh

@@ -33,11 +33,15 @@ rndccmd() {
 }
 
 # Set zone name ($1) and policy ($2) for testing nsec3.
+# Also set the expected number of keys ($3) and DNSKEY TTL ($4).
 set_zone_policy() {
 	ZONE=$1
 	POLICY=$2
 	NUM_KEYS=$3
 	DNSKEY_TTL=$4
+	# The CDS digest type in these tests are all the default,
+	# which is SHA-256 (2).
+	DIGEST_TYPE=2
 }
 # Set expected NSEC3 parameters: flags ($1), iterations ($2), and
 # salt length ($3).