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/iperf3.py b/src/osmo_gsm_tester/iperf3.py
index 7103193..331a3ba 100644
--- a/src/osmo_gsm_tester/iperf3.py
+++ b/src/osmo_gsm_tester/iperf3.py
@@ -20,7 +20,8 @@
 import os
 import json
 
-from . import log, util, config, process, pcap_recorder, run_node, remote
+from .core import log, util, config, process, remote
+from . import pcap_recorder, run_node
 
 def iperf3_result_to_json(file):
     with open(file) as f: