increase filter_abund.py coverage

- [x] Is it mergable
- [x] Did it pass the tests?
- N/A If it introduces new functionality in scripts/ is it tested?
  Check for code coverage.
- [x] Is it well formatted? Look at `pep8`/`pylint`, `cppcheck`, and
  `make doc` output. Use `autopep8` and `astyle -A10 --max-code-length=80`
  if needed.
- [x] Is it documented in the Changelog?
- [x] Was spellcheck run on the source code and documentation after changes
  were made?
On Tue, Sep 30, 2014 at 11:54:53PM -0700, Michael Wright wrote:
> >      assert len(seqs) == 1, seqs
> >      assert 'GGTTGACGGGGCTCAGGG' in seqs
> >  
> > +    args = [n_counting_ht, n_infile]
> > +    utils.runscript(script, args, n_in_dir)
> > +
> > +    seqs = set([r.sequence for r in screed.open(n_infile)])
> > +    assert os.path.exists(n_outfile), n_outfile
> > +
> > +    args = [n_counting_ht, n_infile, '-o' 'outfile']
> > +    utils.runscript(script, args, in_dir)
> > +    assert os.path.exists(outfile), outfile
> 
> @ctb should I use %s or '.format' here?
> http://stackoverflow.com/questions/12382719/python-way-of-printing-with-format-or-percent-form
> (I know stack overflow, sorry. But I couldn't find documentation on my question. I only see it as important if '%' is not supported for python3)

They've been mixed in the codebase, so it doesn't matter.  I gather {} is
more modern.