itest: update channel creation itest with Memo field logic

We test both the happy path (valid memo is returned when querying),
as well as the unhappy path (invalid memo rejects the open action).
To accomplish this, we update the OpenChannelParams struct inside
the harness to accept the Memo.

lntest/harness.go

@@ -890,6 +890,12 @@ type OpenChannelParams struct {
 	// FundMax is a boolean indicating whether the channel should be funded
 	// with the maximum possible amount from the wallet.
 	FundMax bool
+
+	// An optional note-to-self containing some useful information about the
+	// channel. This is stored locally only, and is purely for reference. It
+	// has no bearing on the channel's operation. Max allowed length is 500
+	// characters.
+	Memo string
 }
 
 // prepareOpenChannel waits for both nodes to be synced to chain and returns an
@@ -931,6 +937,7 @@ func (h *HarnessTest) prepareOpenChannel(srcNode, destNode *node.HarnessNode,
 		UseBaseFee:         p.UseBaseFee,
 		UseFeeRate:         p.UseFeeRate,
 		FundMax:            p.FundMax,
+		Memo:               p.Memo,
 	}
 }