iperf3: Avoid creating empty log file

Since a while ago, we avoid by default using the --logfile param to log
the json output to a different file, in order to stay compatible with
older versions of iperf3.
However, the local logfile was still being created, and was not
overwritten with the remote file with content because there's none
created remotely (due to reasons explained above).

Change-Id: I218ec28dc65febc7bd083d6d15854319bf4f3daf
diff --git a/src/osmo_gsm_tester/obj/iperf3.py b/src/osmo_gsm_tester/obj/iperf3.py
index d61dbb1..9e084aa 100644
--- a/src/osmo_gsm_tester/obj/iperf3.py
+++ b/src/osmo_gsm_tester/obj/iperf3.py
@@ -117,7 +117,7 @@
         self.log('Starting iperf3-srv')
         self.log_copied = False
         self.run_dir = util.Dir(self.testenv.test().get_run_dir().new_dir(self.name()))
-        self.log_file = self.run_dir.new_file(IPerf3Server.LOGFILE)
+        self.log_file = self.run_dir.new_child(IPerf3Server.LOGFILE)
         if self.runs_locally():
             self.start_locally()
         else:
@@ -258,7 +258,7 @@
         self.log('Preparing iperf3-client connecting to %s:%d (proto=%s,time=%ds)' % (self.server.addr(), self.server.port(), self._proto, time_sec))
         self.log_copied = False
         self.run_dir = util.Dir(self.testenv.test().get_run_dir().new_dir(self.name()))
-        self.log_file = self.run_dir.new_file(IPerf3Client.LOGFILE)
+        self.log_file = self.run_dir.new_child(IPerf3Client.LOGFILE)
 
         popen_args = ('iperf3', '-c',  self.server.addr(),
                       '-p', str(self.server.port()), '-J',