tests: lnpeer: fix flaky test "hold_invoice_set_doesnt_get_expired"

[SomberNight]

This test was flaky: the mpp_set resolution gets set to SETTLING several asyncio event loop iterations before the hold invoice callback "cb" gets called. If the 0.1 sec polling triggers just in the middle of that interval, assert cb_got_called fails.

    async def check_mpp_state():
        async def wait_for_resolution():
            while True:
                await asyncio.sleep(0.1)
                if payment_key not in bob_w.received_mpp_htlcs:
                    continue
                if not bob_w.received_mpp_htlcs[payment_key].resolution == RecvMPPResolution.SETTLING:
                    continue
                return
        await util.wait_for2(wait_for_resolution(), timeout=2)
>       assert cb_got_called
E       assert False

tests/test_lnpeer.py:1898: AssertionError

see

electrum/electrum/lnpeer.py

Lines 3136 to 3137 in 16c8cb5

	self.lnworker.set_mpp_resolution(payment_key, RecvMPPResolution.SETTLING)
	return None, None, callback

fixes #10589

---

diff to reproduce the failure without present patch:

diff --git a/tests/test_lnpeer.py b/tests/test_lnpeer.py
index https://github.com/SomberNight/electrum/commit/8669931c24fc5ba3015b272b83cfdf0a3c24f067..e15973d68f 100644
--- a/tests/test_lnpeer.py
+++ b/tests/test_lnpeer.py
@@ -1885,6 +1885,7 @@ class TestPeerDirect(TestPeer):
             cb_got_called = False
             async def cb(_payment_hash):
                 self.logger.debug(f"hold invoice callback called. {bob_w.network.get_local_height()=}")
+                await asyncio.sleep(1)
                 nonlocal cb_got_called
                 cb_got_called = True