Add missing newline in util_ChainMerge test

Summary:

This was causing a lot of test cases not not be very meaningful because
multiple configuration options were combined into one line.

The changes in test output with this fix make sense and look like:

diff
- testnet=1 regtest=1 || test
+ testnet=1 regtest=1 || error: Invalid combination of -regtest, -testnet and -chain. Can use at >  > most one.

Issue was reported and debugged by
Wladimir J. van der Laan <laanwj@protonmail.com> in
https://github.com/bitcoin/bitcoin/pull/17385#issuecomment-550033222

This is a backport of Core PR17388

Test Plan: ninja && ninja check

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, majcosta

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D8257