Improve our testing

[jni]

In #11, @trhallam added some basic tests (🎉!). We can be more elaborate:

• [ ] ensure that running the function with a lowlevelcallable and a Python callable (the un-decorated function) gives the same results
• [ ] ensure that lowlevelcallable is faster than pure python
• [ ] ensure that providing input array as output gives the desired results (not sure yet whether this is technically possible from the SciPy side).