Change the way fixtures are injected into tests and convert related tests to class based tests.

[Tomasz-Kluczkowski]

• Convert all related tests into class based tests with appropriate names.
• use the example below to inject required fixtures per class. Remember that this will redo all injected fixutres per each test in a class. If a fixture is needed for only one test among many in the class test do not inject it in the decorator but normally at method level.
  This is to avoid specifying fixture in each method separately as it is now.

@pytest.mark.usefixtures("cleandir")
class TestDirectoryInit(object):
    def test_cwd_starts_empty(self):
        assert os.listdir(os.getcwd()) == []
        with open("myfile", "w") as f:
            f.write("hello")

    def test_cwd_again_starts_empty(self):
        assert os.listdir(os.getcwd()) == []