test: Disable CNAME lookup caching

internal/dns/dns.go

@@ -1,7 +1,6 @@
 package dns
 
 import (
-	"errors"
 	"net"
 	"strings"
 	"time"
@@ -59,10 +58,6 @@ func LookupRepoTXT(domain string) (string, error) {
 func LookupCNAME(domain string) (string, error) {
 	cname, found := cnameCache.Get(domain)
 	if found {
-		if cname == "" {
-			return "", errors.New("Previous request failure")
-		}
-
 		return cname.(string), nil
 	}
 
@@ -72,6 +67,5 @@ func LookupCNAME(domain string) (string, error) {
 		return cname.(string), nil
 	}
 
-	cnameCache.Set(domain, "", cache.DefaultExpiration)
 	return "", err
 }