> This is a pretty interesting approach, but what is the point of it being open at all if it is prohibitively difficult to develop on without tests?

Personally, I wouldn't use a proprietary library/framework unless absolutely necessary. I think it's a great strategy, actually; OP is sacrificing outside contributions while making it much more difficult for someone to just fork the project and bypass them entirely.

I am not sacrificing much. Project was fully open for a while, bot no coauthor emerged. I only get minor patches. Contributions are happening outside core project, some people build pretty awesome stuff on my library, I also get connectors etc...

I would say this approach fits great for one-men projects.

I believe sqlite does the same thing.