[chronik] Electrum: handle multiple messages in a single packet in the test framework

Summary: It is possible for the server to send several '\n' separated messages in a single data frame. This diff adds support for this. For the current tests there is no change in behavior.

Test Plan:

./test/functional/test_runner.py chronik_electrum_*

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Subscribers: PiRK

Differential Revision: https://reviews.bitcoinabc.org/D18097