itest: assert unspent before performing CPFP

This reduces the flakiness of the CPFP test by asserting the wallet has
seen the unspent output before attempting to perform the walletkit's
BumpFee method.

Previously the attempt to bump the fee of the target transaction could
be made before the wallet had had a chance to fully process the
transaction, causing a flaky error.

lnrpc/walletrpc/walletkit_server.go

@@ -678,6 +678,8 @@ func (w *WalletKit) BumpFee(ctx context.Context,
 		return nil, err
 	}
 
+	log.Debugf("Attempting to CPFP outpoint %s", op)
+
 	// Since we're unable to perform a bump through RBF, we'll assume the
 	// user is attempting to bump an unconfirmed transaction's fee rate by
 	// sweeping an output within it under control of the wallet with a