remove unused variables
run Java test as part of the normal testing

Surprisingly that highlights at least two problems:

* attributes in Java at least suffer from some C string termination (autotest thinks the Java test emits binary data). That is true independent of the backend so it is not a problem with the recent changes in HDF5. I think I've seen this way earlier and mostly work around the problem by padding data with zeros.
* the HDF5 library goes into an infinite loop closing the library, unless it was the last library used (even if it wasn't used at all)
fix reading of strings
add extra byte for null termination when reading attributes
include Java tests and make them pass