Tests: make assertions platform-neutral

[fuguiKz]

Fixes #2375

This is tests-only (no behavior change). It makes a few assertions resilient to platform differences:

• Build newline-sensitive expected output using cw.Configuration.NewLine/TestStringBuilder (avoids checkout EOL differences).
• Use an invalid culture name that reliably throws on ICU + Windows NLS.
• Build expected DateTime output via dob.ToString(culture) to handle ICU's narrow no-break space.

Verification (Linux):

• dotnet test tests/CsvHelper.Tests/CsvHelper.Tests.csproj -c Release -f net8.0

[fuguiKz]

Note: the GitHub Actions check suite is currently in an action_required state (fork workflow approval), so CI hasn't executed yet.

Would a maintainer be able to approve and run the workflows for this PR?

This is tests-only (no behavior change). Local verification (Linux): dotnet test tests/CsvHelper.Tests/CsvHelper.Tests.csproj -c Release -f net8.0 ✅