As an open source developer for 3 decades now, I used to have this flippant attitude. Trust me when I say, it doesn't work.

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.