[cdc-composer][tests] Refactor newline handling for cross-platform compatibility

[Cqz666]

The hardcoded '\n' character was causing failures in unit tests on Windows platforms. Replace hardcoded newline character '\n' with 'System.lineSeparator()' to improve code portability across different operating systems.

[Cqz666]

CC @PatrickRen

[Cqz666]

@leonardBang

[yuxiqian]

Thanks for @Cqz666's contribution, could you please rebase this PR with latest master branch before it could be merged?

[github-actions[bot]]

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