pytest: fix flake in test_excluded_adjacent_routehint

Make sure l1 can see l2's channel and it's in the gossip_store.

```
    def test_excluded_adjacent_routehint(node_factory, bitcoind):
        """Test case where we try have a routehint which leads to an adjacent
        node, but the result exceeds our maxfee; we crashed trying to find
        what part of the path was most expensive in that case
    
        """
        l1, l2, l3 = node_factory.line_graph(3)
    
        # We'll be forced to use routehint, since we don't know about l3.
        wait_for(lambda: len(l3.rpc.listchannels(source=l2.info['id'])['channels']) == 1)
        inv = l3.rpc.invoice(10**3, "lbl", "desc", exposeprivatechannels=l2.get_channel_scid(l3))
    
        # This will make it reject the routehint.
        err = r'Fee exceeds our fee budget: 1msat > 0msat, discarding route'
        with pytest.raises(RpcError, match=err):
>           l1.rpc.pay(bolt11=inv['bolt11'], maxfeepercent=0, exemptfee=0)

tests/test_pay.py:3420: 
...
>           raise RpcError(method, payload, resp['error'])
E           pyln.client.lightning.RpcError: RPC call failed: method: pay, payload: {'bolt11': 'lnbcrt10n1pjjm8hesp5kp4nfgrj2ev6dz6xuqgxg29hz7263ltlafylhw7nglhtjxeqpn7spp5w92tjq8a354psfhdzmeuytfc6eye4f5egl7tj7s0f5ftz0k4pmcqdq8v3jhxccxqyjw5qcqp9rzjqgkjyd3q5dv6gllh77kygly9c3kfy0d9xwyjyxsq2nq3c83u5vw4jqqqvuqqqqgqqqqqqqqpqqqqqzsqqc9qxpqysgqetjwr6ql24jrz02qhj7pdw3kqynw6j3sgj2h32ufeyzasjyp2j6yc5durewjjpjy5yqtfgdxmdj52n7jk0ylzk5wudk4ffmjyyw6jmsqkvjex9', 'maxfeepercent': 0, 'exemptfee': 0}, error: {'code': 210, 'message': 'Destination 035d2b1192dfba134e10e540875d366ebc8bc353d5aa766b80c090b39c3a5d885d is not reachable directly and all routehints were unusable.', 'attempts': [{'status': 'failed', 'failreason': 'Destination 035d2b1192dfba134e10e540875d366ebc8bc353d5aa766b80c090b39c3a5d885d is not reachable directly and all routehints were unusable.', 'partid': 0, 'amount_msat': 1000msat}]}
```

Signed-off-by: Rusty Russell <[email protected]>

tests/test_pay.py

@@ -3414,6 +3414,7 @@ def test_excluded_adjacent_routehint(node_factory, bitcoind):
     wait_for(lambda: len(l3.rpc.listchannels(source=l2.info['id'])['channels']) == 1)
     inv = l3.rpc.invoice(10**3, "lbl", "desc", exposeprivatechannels=l2.get_channel_scid(l3))
 
+    wait_for(lambda: len(l1.rpc.listchannels(source=l2.info['id'])['channels']) == 1)
     # This will make it reject the routehint.
     err = r'Fee exceeds our fee budget: 1msat > 0msat, discarding route'
     with pytest.raises(RpcError, match=err):