Updated DiamondTests.cs

[spirit11]

Rows method adjusted to use system dependent new line characters.

Using '\n' instead of Environment.NewLine caused an issue, when all strings except the last one contain '\r' extra character, which leads to property tests failure.

[github-actions]

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[kotp]

Forum discussion is at https://forum.exercism.org/t/c-track-diamond-task-test-fix-suggestion/14056