net/http: fix dns hijacking test

The name lookups are unrooted; the test should be unrooted too.
Correctly skips the tests if the DNS config specifies a domain
suffix that has a wildcard entry causing all unrooted names to resolve.

Change-Id: I80470326a5d332f3b8d64663f765fd304c5e0811
Reviewed-on: https://go-review.googlesource.com/36253
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/net/http/transport_test.go

@@ -3436,7 +3436,7 @@ func skipIfDNSHijacked(t *testing.T) {
 	// DNS server hijacking queries.
 	// See issues 16732, 16716.
 	isDNSHijackedOnce.Do(func() {
-		addrs, _ := net.LookupHost("dns-should-not-resolve.golang.")
+		addrs, _ := net.LookupHost("dns-should-not-resolve.golang")
 		isDNSHijacked = len(addrs) != 0
 	})
 	if isDNSHijacked {