debian/tests/unittests3: use nosetests3 instead of nosetests-3.x.

My yesterday's nose upload to unstable removed `/usr/bin/nosetests-3.x` scripts:

```
nose (1.3.0-2) unstable; urgency=low
  ...
  * Drop nosetests-3.x scripts. There is no easy way to provide these
    scripts for all supported Python 3 versions, and there are other
    ways to do the same thing (like running python3.x -m nose).
```

This pull request removes usage of those removed scripts from the DEP-8 tests.