Hack to get unit tests working from debian packages

This patch modifies the realtime system so that it can run in a
`${DESTDIR}` directory so that unit tests can be run during package
builds, where Machinekit is installed under `${DESTDIR}${prefix}`.
FIXME:  WIP:  cleaning tests

- Clean up groups
  - Document in run_tests.py script
  - Fix dependencies
- Broken attempt at decorator that runs cleanup function
revert back to plain nosetsts
fixup for nosetest-ification

- Add --with-xunit to debian package run
- Fixups:
  - Move utils.py to __init__.py
  - Fix nosetests target in src/Makefile
fixup for nosetests

Fixups:
- Re-enable environment
- Move realtime start/stop into separate tests
FIXME:  wrong branch:  Makefile:  run modsilent.py unbuffered
nosetests fixup:

- Add logging
  - Add hack to `realtime` script to log to a `$RTAPI_LOG` file