Change to run all tests with python -m unittest, which will allow us to
remove workaround for the $PYTHONPATH set by setup.py test.

For this we use `[python-interpreter]` which uses pygolang.main to behave
like a "normal" python interpreter with eggs available, but without the
usual problems of interpreters generated by zc.recipe.eggs - it supports
all command line flags and have current directory in sys.path.

Tests now have an explicit name, that we set to the software name as before,
and the special test dynamically checking all profiles json schemas (part
of slapos.cookbook egg, at the root of slapos repository),is named
json-schemas. It is also changed to run as default tests in python2 and
python3, not only in extras for python2.