Unconsumed data is compressed data. If the part which handles unconsumed data does not work when zlib is available, then the existing tests would fail. In any case the unconsumed buffer is an implementation detail of zipfile.

I see a point in adding a test to make sure zipfile behaves as expected when zlib is not available, but how?

Also, on which systems is zlib missing? I don't see this mentioned in the zlib docs.