pytest: mark test_partial_payment_restart slow.

We can actually be slow enough that we get a (60 second) MPP timeout
under valgrind:

```
        # Restart, and make sure it's reconnected to l2.
        l3.restart()
        print(l2.rpc.listpeers())
        wait_for(lambda: [p['connected'] for p in l2.rpc.listpeers()['peers']] == [True, True])
    
        # Pay second part.
        l1.rpc.sendpay(
            route=route,
            payment_hash=inv['payment_hash'],
            amount_msat=1000,
            bolt11=inv['bolt11'],
            payment_secret=paysecret,
            partid=2,
            groupid=1,
        )
    
>       l1.rpc.waitsendpay(payment_hash=inv['payment_hash'], timeout=TIMEOUT, partid=1)

tests/test_pay.py:3352: 
...
E           pyln.client.lightning.RpcError: RPC call failed: method: waitsendpay, payload: {'payment_hash': 'd73e10b604f53afb05da052d1fc90c45269321169196e31b7ccc312eabc26557', 'timeout': 180, 'partid': 1}, error: {'code': 204, 'message': 'failed: WIRE_MPP_TIMEOUT (reply from remote)', 'data': {'created_index': 1, 'id': 1, 'payment_hash': 'd73e10b604f53afb05da052d1fc90c45269321169196e31b7ccc312eabc26557', 'groupid': 1, 'partid': 1, 'destination': '03cecbfdc68544cc596223b68ce0710c9e5d2c9cb317ee07822d95079acc703d31', 'amount_msat': 500, 'amount_sent_msat': 501, 'created_at': 1771551338, 'status': 'pending', 'bolt11': 'lnbcrt10n1p5e0wnfsp5vpgn0adhmyjecp47yfc6dtefett7d3qqzh4j908ane7tezyy0evqpp56ulppdsy75a0kpw6q5k3ljgvg5nfxggkjxtwxxmuescja27zv4tsdq9d9h8vxqyjw5qcqp9rzjqvuytqpdyk6wqaxvl47d3vee5swuwklej79qxjqqg394r4ptqaue5qqqvuqqqqgqqqqqqqqpqqqqqzsqqc9qxpqysgq6ur4cclfhnzs0tgenmxhvx8glw3q9eylp3pdck73x58cukwtfxj5zjg6g0a4p4plgj6qd0s0uxkuntzcn4l6vvndgrjj7gndma3t7pcqmr9q3j', 'erring_index': 2, 'failcode': 23, 'failcodename': 'WIRE_MPP_TIMEOUT', 'erring_node': '03cecbfdc68544cc596223b68ce0710c9e5d2c9cb317ee07822d95079acc703d31', 'erring_channel': '103x1x0', 'erring_direction': 0, 'raw_message': '0017'}}
```

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

Author: rustyrussell

tests/test_pay.py

@@ -3310,6 +3310,7 @@ def test_partial_payment_timeout(node_factory, bitcoind):
     l2.daemon.wait_for_log(r'HTLC set contains 2 HTLCs, for a total of 1000msat out of 1000msat \(payment_secret\)')
 
 
+@pytest.mark.slow_test
 def test_partial_payment_restart(node_factory, bitcoind):
     """Test that we recover a set when we restart"""
     l1, l2, l3 = node_factory.line_graph(3, wait_for_announce=True,