test: enable testifylint and fix lint issues

• Please check this box if this contribution uses AI-generated content (including content generated by GitLab Duo features) as outlined in the GitLab DCO & CLA. As a benefit of being a GitLab Community Contributor, you receive complimentary access to GitLab Duo.

This MR refactors tests using testifylint.

Most changes were made automatically by running: golangci-lint run --enable-only testifylint --fix ./...