net: use t.Fatalf, not t.Fatal, in test

Found by cmd/vet.

Change-Id: Id570ecd76c3f1efd9696680ccd9799610217f8f7
Reviewed-on: https://go-review.googlesource.com/17042
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/net/net_test.go

@@ -282,5 +282,5 @@ func TestListenCloseListen(t *testing.T) {
 		}
 		t.Errorf("failed on try %d/%d: %v", tries+1, maxTries, err)
 	}
-	t.Fatal("failed to listen/close/listen on same address after %d tries", maxTries)
+	t.Fatalf("failed to listen/close/listen on same address after %d tries", maxTries)
 }