Fix for some tests running twice

All tests from `slapos.test.recipe` module were executed twice since nexedi/slapos@f8de4cc4  when running as `python setup.py test`


`python setup.py test --test-suite slapos.test.test_recipe.additional_tests` still runs only tests for recipes and `python setup.py test` runs tests for recipe and static tests for software releases (for now json schema).

@rafael @katomaso 

/reviewed-on nexedi/slapos!234