Create core directory to contain most of code not in object classes used by tests

Change-Id: I9aec6c55ccd71894182057d36e0025b69925d314
diff --git a/src/osmo_gsm_tester/bts_osmo.py b/src/osmo_gsm_tester/bts_osmo.py
index 9105c28..39ca264 100644
--- a/src/osmo_gsm_tester/bts_osmo.py
+++ b/src/osmo_gsm_tester/bts_osmo.py
@@ -20,7 +20,8 @@
 import os
 import tempfile
 from abc import ABCMeta, abstractmethod
-from . import log, bts, pcu_osmo
+from .core import log
+from . import bts, pcu_osmo
 
 class OsmoBts(bts.Bts, metaclass=ABCMeta):