You don't even have to close. You can just refuse to merge code which does not include 100% test coverage. If someone wants the feature badly enough, they will

You don't even have to close. You can just refuse to merge code which does not include 100% test coverage. If someone wants the feature badly enough, they will figure out a way to fill the gap. Alternatively, someone can always fork the code and release "OpenSSL-but-with-lots-of-untested-code" variant.