Tests: isolation check moved to the pytest_sessionstart().

This change eliminates the need for some classes
to run Unit one more time before running tests.

test/test_python_isolation.py

@@ -2,30 +2,13 @@ import shutil
 
 import pytest
 
-from conftest import unit_run
-from conftest import unit_stop
 from unit.applications.lang.python import TestApplicationPython
-from unit.feature.isolation import TestFeatureIsolation
 from unit.option import option
 
 
 class TestPythonIsolation(TestApplicationPython):
     prerequisites = {'modules': {'python': 'any'}, 'features': ['isolation']}
 
-    @classmethod
-    def setup_class(cls, complete_check=True):
-        check = super().setup_class(complete_check=False)
-
-        unit = unit_run()
-        option.temp_dir = unit['temp_dir']
-
-        TestFeatureIsolation().check(option.available, unit['temp_dir'])
-
-        assert unit_stop() is None
-        shutil.rmtree(unit['temp_dir'])
-
-        return check if not complete_check else check()
-
     def test_python_isolation_rootfs(self, is_su, temp_dir):
         isolation_features = option.available['features']['isolation'].keys()