lntest/harness: adds WaitInvariant helper method

This commits adds a complement to the existing WaitPredicate
helper function, ensuring that a boolean statement remains
true for the duration of the provided timeout. This expands
our ability to do simple sanity checks where the wait-until
semantics of WaitPredicate may not be as-well suited.

lntest/harness.go

@@ -846,6 +846,32 @@ func WaitPredicate(pred func() bool, timeout time.Duration) error {
 	}
 }
 
+// WaitInvariant is a helper test function that will wait for a timeout period
+// of time, verifying that a statement remains true for the entire duration.
+// 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 maintained over a particular time
+// frame.
+func WaitInvariant(statement func() bool, timeout time.Duration) error {
+	const pollInterval = 20 * time.Millisecond
+
+	exitTimer := time.After(timeout)
+	for {
+		<-time.After(pollInterval)
+
+		// Fail if the invariant is broken while polling.
+		if !statement() {
+			return fmt.Errorf("invariant broken before time out")
+		}
+
+		select {
+		case <-exitTimer:
+			return nil
+		default:
+		}
+	}
+}
+
 // DumpLogs reads the current logs generated by the passed node, and returns
 // the logs as a single string. This function is useful for examining the logs
 // of a particular node in the case of a test failure.