net: use .invalid for an invalid domain name

Fixes #25370

Change-Id: I12da0cc17f433ca12c85fb986d65ac9ecb2c3f20
Reviewed-on: https://go-review.googlesource.com/c/151359
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/net/lookup_test.go

@@ -925,7 +925,7 @@ func TestLookupHostCancel(t *testing.T) {
 
 	const (
 		google        = "www.google.com"
-		invalidDomain = "nonexistentdomain.golang.org"
+		invalidDomain = "invalid.invalid" // RFC 2606 reserves .invalid
 		n             = 600               // this needs to be larger than threadLimit size
 	)