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/conftest.py

@@ -17,6 +17,7 @@ import pytest
 from unit.check.go import check_go
 from unit.check.node import check_node
 from unit.check.tls import check_openssl
+from unit.check.isolation import check_isolation
 from unit.option import option
 from unit.utils import public_dir
 from unit.utils import waitforfiles
@@ -123,6 +124,7 @@ def pytest_sessionstart(session):
     option.available = {'modules': {}, 'features': {}}
 
     unit = unit_run()
+    option.temp_dir = unit['temp_dir']
 
     # read unit.log
 
@@ -161,6 +163,8 @@ def pytest_sessionstart(session):
         k: v for k, v in option.available['modules'].items() if v is not None
     }
 
+    check_isolation()
+
     unit_stop()
 
     _check_alerts()