tests: add an xpay test to reproduce a regression where we pay fees in a direct route

[daywalker90]

I noticed this in one of my plugin's tests.

        # BOLT 12, direct peer
        offer = l2.rpc.offer('any')['bolt12']
        b12 = l1.rpc.fetchinvoice(offer, '10000msat')['invoice']
        ret = l1.rpc.xpay(invstring=b12)
        assert ret['failed_parts'] == 0
        assert ret['successful_parts'] == 1
        assert ret['amount_msat'] == 10000
>       assert ret['amount_sent_msat'] == 10000
E       assert 10001 == 10000

tests/test_xpay.py:187: AssertionError

This is also quite flaky on my local tests with this result instead:

>       ret = l1.rpc.xpay(invstring=b12)

tests/test_xpay.py:183:

pyln.client.lightning.RpcError: RPC call failed: method: xpay, payload: {'invstring': '<removed>'}, error: {'code': 205, 'message': 'Failed: We could not find a usable set of paths. The shortest path is 103x1x0->103x2x0->0x0x0, but 0x0x0/1 exceeds htlc_maximum_msat ~0msat'}

[daywalker90]

This also happens with pay btw with even weirder results:

l1-cli pay "<removed>"
{
   "destination": "035ed467e11a1990fd4fc7d98dafbc3c49abcf8d2a48fb24f914e909e95a6737cb",
   "payment_hash": "b461b5fae962473ad0d198c0a69e458cbd6a9ef959918ea5cd6c434e75422ff4",
   "created_at": 1774304621.999080537,
   "parts": 1,
   "amount_msat": 10001,
   "amount_sent_msat": 10001,
   "payment_preimage": "a70a45a55d5121b9ba54daeb62a8cb0bb31fcaeda643974ef3ed8952c0c83f55",
   "status": "complete"
}

This is a direct pay from l1 to l2 with an "any" offer aswell and a fetched invoice for 10000msat. Notice how both amount_msat and amount_sent_msat include a fee.