chore: modernize tests according to staticheck hints (#28)

No functional change.

Author: bassosimone

resolver_test.go

@@ -310,12 +310,13 @@ func TestResolver_LookupHost(t *testing.T) {
 			mockQuery: func(ctx context.Context, addr *ServerAddr, query *dns.Msg) (*dns.Msg, error) {
 				msg := &dns.Msg{}
 				msg.SetReply(query)
-				if query.Question[0].Qtype == dns.TypeA {
+				switch query.Question[0].Qtype {
+				case dns.TypeA:
 					msg.Answer = append(msg.Answer, &dns.A{
 						Hdr: dns.RR_Header{Name: query.Question[0].Name, Rrtype: dns.TypeA, Class: dns.ClassINET, Ttl: 300},
 						A:   net.ParseIP("192.0.2.1"),
 					})
-				} else if query.Question[0].Qtype == dns.TypeAAAA {
+				case dns.TypeAAAA:
 					msg.Answer = append(msg.Answer, &dns.AAAA{
 						Hdr:  dns.RR_Header{Name: query.Question[0].Name, Rrtype: dns.TypeAAAA, Class: dns.ClassINET, Ttl: 300},
 						AAAA: net.ParseIP("2001:db8::1"),

response_test.go

@@ -140,11 +140,12 @@ func TestRCodeToError(t *testing.T) {
 			resp := new(dns.Msg)
 			resp.Rcode = tt.rcode
 
-			if tt.name == "LameReferral" {
+			switch tt.name {
+			case "LameReferral":
 				resp.Authoritative = false
 				resp.RecursionAvailable = false
 				resp.Answer = nil
-			} else if tt.name == "Success" {
+			case "Success":
 				resp.Authoritative = true
 				resp.RecursionAvailable = true
 				resp.Answer = []dns.RR{&dns.A{