If it’s a useful library, you want to be more and more certain it doesn’t have bugs, so yes, you keep adding tests. Maybe you stop when it’s clear that the library is perfect. But when has any project reached that state?

If it’s too small though, it’s probably not that useful and this doesn’t apply. It doesn’t really make sense to care about whether it was abandoned, either, because it will be so small that it doesn’t have any onboarding time and anyone can pick it up at any time.