lightningd: injectpaymentonion can use scids of unannounced channels.

Cut & paste from the forwarding code, where we don't let onions use the
unannounced scids.  Obviously local commands can use them.

Reported-by: @michael1011
Changelog-Fixed: JSON-RPC: xpay now works through unannounced channels.

lightningd/pay.c

@@ -1985,7 +1985,7 @@ static struct command_result *json_injectpaymentonion(struct command *cmd,
 	if (payload->forward_channel) {
 		next = any_channel_by_scid(cmd->ld,
 					   *payload->forward_channel,
-					   false);
+					   true);
 		if (!next)
 			return command_fail(cmd, JSONRPC2_INVALID_PARAMS,
 					    "Unknown scid %s",

tests/test_xpay.py

@@ -532,7 +532,6 @@ def test_xpay_maxfee(node_factory, bitcoind, chainparams):
     assert fee <= maxfee
 
 
-@pytest.mark.xfail(strict=True)
 def test_xpay_unannounced(node_factory):
     l1, l2 = node_factory.line_graph(2, announce_channels=False)