Trend unit tests

[MarkBaker]

This is:

- [ ] a bugfix
- [ ] a new feature
- [X] refactoring and testing

Checklist:

• Changes are covered by unit tests
• Code style is respected
• Commit message explains why the change is made (see https://github.com/erlang/otp/wiki/Writing-good-commit-messages)
• CHANGELOG.md contains a short summary of the change
• Documentation is updated as necessary

Why this change is needed?

Improving code quality and unit tests

[PowerKiKi]

Converting to draft according to Mark in https://gitter.im/PHPOffice/PhpSpreadsheet?at=6060641bf07ffa1eb56f0b33

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs.
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