make the dispute test run by itself

It's kind of slow.

node/node_test.go

@@ -200,6 +200,9 @@ func TestPaymentChannels(t *testing.T) {
 }
 
 func TestWindowPostDispute(t *testing.T) {
+	if os.Getenv("LOTUS_TEST_WINDOW_POST") != "1" {
+		t.Skip("this takes a few minutes, set LOTUS_TEST_WINDOW_POST=1 to run")
+	}
 	logging.SetLogLevel("miner", "ERROR")
 	logging.SetLogLevel("chainstore", "ERROR")
 	logging.SetLogLevel("chain", "ERROR")