[chronik-client] Fix a bug in the plugins integration test

Summary:
The test expects the transactions to be sorted by txid, which is not always true as some mempool endpoints return the txs sorted by time first seen then txid.
Fix this by setting the node mocktime, which makes all the time first seen equal and sorting by txid correct.

Test Plan:

./contrib/teamcity/build-configurations.py chronik-client-integration-tests

Reviewers: #bitcoin_abc, hazzarust, bytesofman

Reviewed By: #bitcoin_abc, hazzarust, bytesofman

Differential Revision: https://reviews.bitcoinabc.org/D17680