Changes in this PR

added some more test coverage to fold subpackage.

Why are you making these changes?

To increase code coverage of fold package.

Are any changes breaking? (IMPORTANT)

No

Pre-merge checklist

All of these must be satisfied before this PR is considered
ready for merging. Mergeable PRs will be prioritized for review.

• New packages/exported functions have docstrings.
• New/changed functionality is thoroughly tested.
• New/changed functionality has a function giving an example of its usage in the associated test file. See primers/primers_test.go for what this might look like.
• Changes are documented in CHANGELOG.md in the [Unreleased] section.
• All code is properly formatted and linted.
• The PR template is filled out.