try fo fix mpp unittests flakiness by waiting an extra iteration

tests/test_lnpeer.py

@@ -1765,6 +1765,10 @@ class TestPeerForwarding(TestPeer):
             if result:
                 self.assertEqual(PR_PAID, dave_w.get_payment_status(lnaddr.paymenthash))
                 # check mpp is cleaned up
+                async with OldTaskGroup() as g:
+                    for peer in peers:
+                        await g.spawn(peer.wait_one_htlc_switch_iteration())
+                # wait another iteration
                 async with OldTaskGroup() as g:
                     for peer in peers:
                         await g.spawn(peer.wait_one_htlc_switch_iteration())