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.