3645e4c Add missing newline in util_ChainMerge test (Russell Yanofsky)

Pull request description:

  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 <[email protected]> in
  #17385 (comment)

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ACKs for top commit:
  laanwj:
    ACK 3645e4c
  practicalswift:
    ACK 3645e4c -- diff looks correct

Tree-SHA512: ca5bde9b9f553811d4827113f4880d15d7b8f4f1455b95bbf34c9a1512fdd53062f1a2133c50d9b54f94160a1ee77a54bc82681a5f3bf25d2b0d01f8a8e95165

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 <[email protected]> in
> bitcoin/bitcoin#17385 (comment)

This is a backport of Core [[bitcoin/bitcoin#17388 | 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