net/http: fix test failure on some Windows machines

The network connection dies differently from the server's
perspective on (some) Windows when the client goes away. Match
on the common prefix (common to Unix and Windows) instead of
the network error part.

Fixes #7456

LGTM=josharian
R=golang-codereviews, josharian
CC=alex.brainman, golang-codereviews, iant
https://golang.org/cl/70010050

src/pkg/net/http/client_test.go

@@ -671,8 +671,8 @@ func TestClientWithIncorrectTLSServerName(t *testing.T) {
 	}
 	select {
 	case v := <-errc:
-		if !strings.Contains(v, "bad certificate") {
-			t.Errorf("expected an error log message containing 'bad certificate'; got %q", v)
+		if !strings.Contains(v, "TLS handshake error") {
+			t.Errorf("expected an error log message containing 'TLS handshake error'; got %q", v)
 		}
 	case <-time.After(5 * time.Second):
 		t.Errorf("timeout waiting for logged error")