add mechanism to block htlcs from settling back

tests/test_lnpeer.py

@@ -197,6 +197,7 @@ class MockLNWallet(Logger, EventListener, NetworkRetryManager[LNPeerAddr]):
         self.preimages = {}
         self.stopping_soon = False
         self.downstream_to_upstream_htlc = {}
+        self.dont_settle_htlcs = {}
         self.hold_invoice_callbacks = {}
         self.payment_bundles = [] # lists of hashes. todo:persist
         self.config.INITIAL_TRAMPOLINE_FEE_LEVEL = 0