tapchannel: add AuxInvoiceManager prop test btc invoice protection

In this commit we extend the property based tests to account for the new
piece of AuxInvoiceManager behavior. If the randomly generated invoice
is not an asset invoice, but the HTLCs do carry assets, we should expect
the manager to reject the HTLCs.

tapchannel/aux_invoice_manager_test.go

@@ -189,6 +189,11 @@ func (m *mockHtlcModifierProperty) HtlcModifier(ctx context.Context,
 			if r.ExitHtlcAmt != res.AmtPaid {
 				m.t.Errorf("AmtPaid != ExitHtlcAmt")
 			}
+		} else if !isAssetInvoice(r.Invoice, m) {
+			if !res.CancelSet {
+				m.t.Errorf("expected cancel set flag")
+			}
+			continue
 		}
 
 		htlcBlob, err := r.WireCustomRecords.Serialize()