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

Change-Id: I9aec6c55ccd71894182057d36e0025b69925d314
diff --git a/selftest/_prep.py b/selftest/_prep.py
index 3ef6e81..582b8fb 100644
--- a/selftest/_prep.py
+++ b/selftest/_prep.py
@@ -7,11 +7,10 @@
 # to find the osmo_gsm_tester py module
 sys.path.append(src_dir)
 
-from osmo_gsm_tester import log
+from osmo_gsm_tester.core import log
 
 log.TestsTarget()
 log.set_all_levels(log.L_DBG)
 
 if '-v' in sys.argv:
     log.style_change(trace=True)
-