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

Change-Id: I9aec6c55ccd71894182057d36e0025b69925d314
diff --git a/selftest/trial_test.py b/selftest/trial_test.py
index ba3f01b..e73ca37 100755
--- a/selftest/trial_test.py
+++ b/selftest/trial_test.py
@@ -3,7 +3,7 @@
 import time
 import _prep
 import os
-from osmo_gsm_tester import util
+from osmo_gsm_tester.core import util
 from osmo_gsm_tester.trial import Trial
 
 workdir = util.get_tempdir()