Fix issue 527

I'm a little cautious that my first PR is to amend the unit tests of khmer, but I believe my dev environment is valid and yet was still failing in specific parts of ```make test```.

The hashbits related changes appear fairly vanilla and were perhaps caused by a data structure change.
The amendment to the output of certain test harness data in ```test_count_median()``` is something I'd highlight for detailed peer review by existing project devs.