[chronik] Improve connection of the electrum interface test

Summary: It is possible that the electrum interface is not ready after a node restart, so let's retry up to a timeout.

Test Plan:

./test/functional/test_runner.py chronik_electrum_*

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D17910