Set ServerName explicitly in TLSConfig to ensure SNI works

Sometime since I first wrote the ACME (Let's Encrypt) code,
and today, golang needs this extra ServerName to be set explicitly
in the TLSConfig.

This is to avoid domain fronting.

golang/go#22704

testutils/utils.go

@@ -131,7 +131,10 @@ func MakeRequest(url string, opts ...ReqOption) (*http.Response, []byte, error)
 	if strings.HasPrefix(url, "https") {
 		tr = &http.Transport{
 			DisableKeepAlives: true,
-			TLSClientConfig:   &tls.Config{InsecureSkipVerify: true},
+			TLSClientConfig:   &tls.Config{
+				InsecureSkipVerify: true,
+				ServerName: request.Host,
+			},
 		}
 	} else {
 		tr = &http.Transport{