lntest: add new WaitPredicate helper function

lntest/harness.go

@@ -773,11 +773,24 @@ func (n *NetworkHarness) AssertChannelExists(ctx context.Context,
 
 	for _, channel := range resp.Channels {
 		if channel.ChannelPoint == chanPoint.String() {
+// WaitPredicate is a helper test function that will wait for a timeout period
+// of time until the passed predicate returns true. This function is helpful as
+// timing doesn't always line up well when running integration tests with
+// several running lnd nodes. This function gives callers a way to assert that
+// some property is upheld within a particular time frame.
+func WaitPredicate(pred func() bool, timeout time.Duration) error {
+	exitTimer := time.After(timeout)
+	for {
+		select {
+		case <-exitTimer:
+			return fmt.Errorf("predicate not satisfied after time out")
+		default:
+		}
+
+		if pred() {
 			return nil
 		}
 	}
-
-	return fmt.Errorf("channel not found")
 }
 
 // DumpLogs reads the current logs generated by the passed node, and returns