Add a unit test for DuplicateCNAME (#329)

This fix adds a unit test for DuplicateCNAME to have
a basic coverage for dnsutil.
Signed-off-by: Yong Tang <yong.tang.github@outlook.com>

middleware/pkg/dnsutil/cname_test.go

+package dnsutil
+
+import (
+	"testing"
+
+	"github.com/miekg/dns"
+)
+
+func TestDuplicateCNAME(t *testing.T) {
+	tests := []struct {
+		cname    string
+		records  []string
+		expected bool
+	}{
+		{
+			"1.0.0.192.IN-ADDR.ARPA.	3600	IN	CNAME	1.0.0.0.192.IN-ADDR.ARPA.",
+			[]string{
+				"US.    86400	IN	NSEC	0-.us. NS SOA RRSIG NSEC DNSKEY TYPE65534",
+				"1.0.0.192.IN-ADDR.ARPA.	3600	IN	CNAME	1.0.0.0.192.IN-ADDR.ARPA.",
+			},
+			true,
+		},
+		{
+			"1.0.0.192.IN-ADDR.ARPA.	3600	IN	CNAME	1.0.0.0.192.IN-ADDR.ARPA.",
+			[]string{
+				"US.    86400	IN	NSEC	0-.us. NS SOA RRSIG NSEC DNSKEY TYPE65534",
+			},
+			false,
+		},
+		{
+			"1.0.0.192.IN-ADDR.ARPA.	3600	IN	CNAME	1.0.0.0.192.IN-ADDR.ARPA.",
+			[]string{},
+			false,
+		},
+	}
+	for i, test := range tests {
+		cnameRR, err := dns.NewRR(test.cname)
+		if err != nil {
+			t.Fatal("Test %d, cname ('%s') error (%s)!", i, test.cname, err)
+		}
+		cname := cnameRR.(*dns.CNAME)
+		records := []dns.RR{}
+		for j, r := range test.records {
+			rr, err := dns.NewRR(r)
+			if err != nil {
+				t.Fatal("Test %d, record %d ('%s') error (%s)!", i, j, r, err)
+			}
+			records = append(records, rr)
+		}
+		got := DuplicateCNAME(cname, records)
+		if got != test.expected {
+			t.Errorf("Test %d, expected '%v', got '%v' for CNAME ('%s') and RECORDS (%v)", i, test.expected, got, test.cname, test.records)
+		}
+	}
+}