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/nitb_osmo.py b/src/osmo_gsm_tester/nitb_osmo.py
index e161ccb..ec4912f 100644
--- a/src/osmo_gsm_tester/nitb_osmo.py
+++ b/src/osmo_gsm_tester/nitb_osmo.py
@@ -21,7 +21,8 @@
 import re
 import pprint
 
-from . import log, util, config, template, process, osmo_ctrl, pcap_recorder, smsc
+from .core import log, util, config, template, process
+from . import osmo_ctrl, pcap_recorder, smsc
 
 class OsmoNitb(log.Origin):