tests/external: Update list of external resolvers

This patch updates the list of external resolvers we test against;
removing some that we see returning regular errors and causing
flakiness, and updating others who have changed.

tests/external.sh

@@ -219,9 +219,8 @@ for server in \
 	"https://1.1.1.1/dns-query" \
 	"https://cloudflare-dns.com/dns-query" \
 	"https://dns.google/dns-query" \
-	"https://dns.quad9.net/dns-query" \
-	"https://doh.nl.ahadns.net/dns-query" \
-	"https://dns.alidns.com/dns-query" \
+	"https://9.9.9.9/dns-query" \
+	"https://blitz.ahadns.com" \
 	"https://odvr.nic.cz/dns-query" \
 	;
 do