pytest: work around pay flakiness.
pay sometimes ignores exclusions. WONTFIX.
```
with pytest.raises(RpcError, match=r'is not reachable directly and all routehints were unusable.'):
> l1.rpc.pay(inv, exclude=[scid12])
tests/test_pay.py:5279:
...
elif "error" in resp:
> raise RpcError(method, payload, resp['error'])
E pyln.client.lightning.RpcError: RPC call failed: method: pay, payload: {'bolt11': 'lnbcrt1230n1p54mma3sp5x7uerjgyg7ws6fnzdwxc7pgpj6j25uhpqp5uvx3fk8dkcqm37m2spp5k02racjc9knux958u5rgtva24jfvxtr5w3t53pfeavn3thmyny0qdq8v3jhxccxqyjw5qcqp9rzjqgkjyd3q5dv6gllh77kygly9c3kfy0d9xwyjyxsq2nq3c83u5vw4jqqqvuqqqqgqqqqqqqqpqqqqqzsqqc9qxpqysgqcmv875mmzcjl8mwxxndy9an6p870ffpdxdtypmgf5gzsydnt2d68n4kjph0rcprye6tfz0ex0c5clgj3zwm8jgd5vs0fdv7hf7dqr8cqdrg3gf', 'exclude': ['103x2x0/1']}, error: {'code': 210, 'message': 'Ran out of routes to try after 6 attempts: see `paystatus`', 'attempts': [{'status': 'failed', 'failreason': 'No path found', 'partid': 0, 'amount_msat': 123000}, {'status': 'pending', 'failreason': 'No path found', 'partid': 1, 'amount_msat': 123000, 'parent_partid': 0}, {'status': 'failed', 'failreason': 'No path found', 'partid': 2, 'amount_msat': 57006, 'parent_partid': 1}, {'status': 'failed', 'failreason': 'No path found', 'partid': 4, 'amount_msat': 57006, 'parent_partid': 2}, {'status': 'failed', 'failreason': 'No path found', 'partid': 3, 'amount_msat': 65994, 'parent_partid': 1}, {'status': 'failed', 'failreason': 'No path found', 'partid': 5, 'amount_msat': 65994, 'parent_partid': 3}]}
```
The logs show that it doesn't exclude the routehint early: in successful runs we get "After filtering routehints we're left with 0 usable hints". Perhaps this is something to do with the timing of our own notifications?
```
2026-01-07T05:51:10.7902502Z lightningd-1 2026-01-07T05:31:29.706Z DEBUG plugin-pay: cmd -c:pay#64/cln:pay#121 partid 0: Received getchaininfo blockcount=108, headercount=108
2026-01-07T05:51:10.7903334Z lightningd-1 2026-01-07T05:31:29.715Z DEBUG plugin-pay: cmd -c:pay#64/cln:pay#121 partid 0: waitblockheight reports syncheight=108
2026-01-07T05:51:10.7904256Z lightningd-1 2026-01-07T05:31:29.734Z DEBUG plugin-pay: cmd -c:pay#64/cln:pay#121 partid 0: Updated a channel hint for 103x2x0/1: enabled true, estimated capacity 978718000msat
2026-01-07T05:51:10.7905355Z lightningd-1 2026-01-07T05:31:29.734Z DEBUG plugin-pay: cmd -c:pay#64/cln:pay#121 partid 0: Updated a channel hint for 7269357x11669990x33910/1: enabled false, estimated capacity UNKNOWN
2026-01-07T05:51:10.7906580Z lightningd-1 2026-01-07T05:31:29.735Z DEBUG plugin-pay: cmd -c:pay#64/cln:pay#121 partid 0: Updated a channel hint for 103x2x0/1: enabled false, estimated capacity UNKNOWN
2026-01-07T05:51:10.7907665Z lightningd-1 2026-01-07T05:31:29.735Z INFO plugin-pay: cmd -c:pay#64/cln:pay#121 partid 0: Payment fee constraint 615msat is below exemption threshold, allowing a maximum fee of 5000msat
2026-01-07T05:51:10.7908845Z lightningd-1 2026-01-07T05:31:29.752Z DEBUG plugin-pay: Received a channel_hint {.scid = 103x2x0/1, .enabled = 1, .estimate = 978718000msat, .capacity = 1000000000msat }
2026-01-07T05:51:10.7909710Z lightningd-1 2026-01-07T05:31:29.754Z INFO plugin-pay: cmd -c:pay#64/cln:pay#121 partid 0: Filtering out 1 routehints
2026-01-07T05:51:10.7910544Z lightningd-1 2026-01-07T05:31:29.779Z DEBUG plugin-pay: cmd -c:pay#64/cln:pay#121 partid 0: Checking hint {.scid=103x2x0/1, .enabled=1, .estimate=978718000msat}
2026-01-07T05:51:10.7911470Z lightningd-1 2026-01-07T05:31:29.780Z DEBUG plugin-pay: cmd -c:pay#64/cln:pay#121 partid 0: After filtering routehints we're left with 1 usable hints
2026-01-07T05:51:10.7912385Z lightningd-1 2026-01-07T05:31:29.780Z DEBUG plugin-pay: cmd -c:pay#64/cln:pay#121 partid 0: Checking hint {.scid=103x2x0/1, .enabled=1, .estimate=978718000msat}
2026-01-07T05:51:10.7913471Z lightningd-1 2026-01-07T05:31:29.780Z DEBUG plugin-pay: cmd -c:pay#64/cln:pay#121 partid 0: Using routehint 022d223620a359a47ff7f7ac447c85c46c923da53389221a0054c11c1e3ca31d59 (103x1x0) cltv_delta=6
```
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
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