Use nxdtest in SlapOS tests

 * switch software/slapos-testing (`SlapOS.Eggs.UnitTest-*`) to use nxdtest and update README to describe how to run tests locally with nxdtest's runTestSuite. Switch tests to `python -m unittest` (or slight variations) instead of the deprecated `python setup.py`, except a few tests which only pass when invoked with `setup.py`.
 * switch software/slapos-sr-testing (`SlapOS.SoftwareReleases.IntegrationTest-*`) to use nxdtest and update README to describe how to run tests locally with nxdtest's runTestSuite. Switch all tests to `python -m unittest discover` intead of the deprecated `python setup.py` - which was especially problematic in these tests, because it sets `$PYTHONPATH` which affect subprocesses ran by the tests. In follow up commits we'll drop the workarounds for `$PYTHONPATH`.

`EggTestSuite` becomes unused and will be removed in follow up changes.

See merge request !862