Test engineers set up automated test suites, run structured manual testing before a release, and build the CI checks that stop a regression reaching your customers.
Testing is bought either as a one-off (a release is coming and nobody has checked it properly) or as an investment (the team is afraid to deploy). The second is where a freelancer adds the most durable value, because the test suite outlives the engagement.
Be realistic about coverage. Complete coverage is neither achievable nor worth paying for; agreeing which flows must never break is.
A short checklist that makes the quotes you receive comparable and the work easier to run.
Is automated testing worth it for a small product? For the few flows that must not break, yes — the suite pays for itself the first time it catches a regression before a customer does. Full coverage of a small product rarely is.
Can the same person write and test the code? They can, and for small projects they usually do. A separate pair of eyes finds a different class of problem, which is worth buying before a significant release.
How long does it take to set up a test suite? Getting the framework running and the first critical journeys covered is usually a small, well-scoped engagement. Growing the suite is ongoing work.
Post the project and let matching freelancers come to you, or approach someone from the list above directly.