Feel free to help :)
Build the framework for tests and then require anyone who wants to help build the product to write tests with their PRs.
You can't just push some code out there and expect people to "feel free to help", it doesn't happen, and is quite a turnoff.
To the downvoters, this is what I see as valid feedback to a rather flippant response.
I've got a project that has been going for 6 years now and attracted 500 stars and gets 49k downloads a month. It works because it has comprehensive unit tests and people can rely on it. When I was just starting out on that project, I didn't tell people to feel free to help. I put the effort in. It is important to lay the groundwork beyond just writing the utility.
I made a lot of mistakes along the way and one of them was being flippant on my responses to people like that. Just sharing my insights.