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/trial.py b/src/osmo_gsm_tester/trial.py
index 149d34c..9dcc188 100644
--- a/src/osmo_gsm_tester/trial.py
+++ b/src/osmo_gsm_tester/trial.py
@@ -22,7 +22,8 @@
 import shutil
 import tarfile
 
-from . import log, util, suite, report
+from .core import log, util
+from . import suite, report
 
 FILE_MARK_TAKEN = 'taken'
 FILE_CHECKSUMS = 'checksums.md5'