Fix flaky aggregated HTLC revocation test.

Fix flaky aggregated HTLC revocation test.

Releasing write locks in between monitor updates
requires storing a set of cloned keys to iterate
over. For efficiency purposes, that set of keys
is an actual set, as opposed to array, which means
that the iteration order may not be consistent.

The test was relying on an event array index to
access the revocation transaction. We change that
to accessing a hash map keyed by the txid, fixing
the test.