testing/testcase: isolate logs snapshot per test class

tell supervisor to clear log so that we don't get the logs from the
previous test. This also will reduce the size of dumps.

slapos/testing/testcase.py

@@ -401,6 +401,8 @@ class SlapOSInstanceTestCase(unittest.TestCase):
     """Tear down class, stop the processes and destroy instance.
     """
     cls._cleanup("{}.{}.tearDownClass".format(cls.__module__, cls.__name__))
+    with cls.slap.system_supervisor_rpc as supervisor:
+      supervisor.clearAllProcessLogs()
 
   @classmethod
   def _storePartitionSnapshot(cls, name):