follow-up fix tests: logic typo

follow-up https://github.com/spesmilo/electrum/pull/7202

defaultdict[int] is a type!

```
>>> from collections import defaultdict
>>> d = defaultdict[int]
>>> d[2]
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: There are no type variables left in collections.defaultdict[int]
```

Also, prior to py3.9, it is a TypeError.

electrum/tests/test_lnpeer.py

@@ -205,7 +205,7 @@ class MockLNWallet(Logger, NetworkRetryManager[LNPeerAddr]):
             min_cltv_expiry=decoded_invoice.get_min_final_cltv_expiry(),
             r_tags=decoded_invoice.get_routing_info('r'),
             invoice_features=decoded_invoice.get_features(),
-            trampoline_fee_levels=defaultdict[int],
+            trampoline_fee_levels=defaultdict(int),
             use_two_trampolines=False,
             payment_hash=decoded_invoice.paymenthash,
             payment_secret=decoded_invoice.payment_secret,