Check, that test are not joined in classess

[Melevir]

In unittest tests are joined inside TestCase class. This is unittest practice, not pytest.

We should check, that all defs inside test files are in root, not inside class definition.

[cleder]

@Melevir pytestmay have test classes as well,

class TestFeature:
    def setup_method(self):

[Melevir]

This is not known as common pytest practice, so it makes sense to validate it.

Anyway, I'm not maintaining the project anymore. All questions and proposals are up to @korneevm :)