[chronik] Don't ever use mainnet on tests

Summary:
This is known to cause issues, and will conflict with any bitcoind instance running on the machine.
The removed test case is trivial enough that it's not a big deal.

Test Plan:

./test/functional/test_runner.py chronik_pause

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D15706