refactor: this PR adds the testutil folder to the linter configuration

[faddat]

Description

This PR removes the exclusion for the testutil folder, so that it is linted in the same way as the rest of this repository.

---

Author Checklist

All items are required. Please add a note to the item if the item is not applicable and
please add links to any relevant follow up issues.

I have...

• Included the correct type prefix in the PR title
• Added ! to the type prefix if state-machine breaking change (i.e., requires coordinated upgrade)
• Confirmed this PR does not introduce changes requiring state migrations, OR migration code has been added to consumer and/or provider modules
• Targeted the correct branch (see PR Targeting)
• Provided a link to the relevant issue or specification
• Followed the guidelines for building SDK modules
• Included the necessary unit and integration tests
• Added a changelog entry to CHANGELOG.md
• Included comments for documenting Go code
• Updated the relevant documentation or specification
• Reviewed "Files changed" and left comments if necessary
• Confirmed all CI checks have passed

Reviewers Checklist

All items are required. Please add a note if the item is not applicable and please add
your handle next to the items reviewed if you only reviewed selected items.

I have...

• confirmed the correct type prefix in the PR title
• confirmed ! in the type prefix if API or client breaking change
• confirmed this PR does not introduce changes requiring state migrations, OR confirmed migration code has been added to consumer and/or provider modules
• confirmed all author checklist items have been addressed
• reviewed state machine logic
• reviewed API design and naming
• reviewed documentation is accurate
• reviewed tests and test coverage

[shaspitz]

@faddat please run make lint or make format so the golangci-lint test can pass. Then this is ready to merge, thanks

[faddat]

@smarshall-spitzbart hey sorry about the delay, was out having a very very cute baby. I think this is g2g now.