LNWorker: give up payment after timeout, not number of attempts.

Limiting attempts may interrupt a MPP before we receive a MPP_timeout
The attempts parameter is still used in unit tests.

electrum/tests/test_lnpeer.py

@@ -117,6 +117,7 @@ class MockWallet:
 
 class MockLNWallet(Logger, NetworkRetryManager[LNPeerAddr]):
     MPP_EXPIRY = 2  # HTLC timestamps are cast to int, so this cannot be 1
+    PAYMENT_TIMEOUT = 120
     TIMEOUT_SHUTDOWN_FAIL_PENDING_HTLCS = 0
     INITIAL_TRAMPOLINE_FEE_LEVEL = 0