process: Move standalone run_local_sync as method of Process

Change-Id: Ib2b3fd39db5400a93a8caabae367dac3e3250247
diff --git a/src/osmo_gsm_tester/iperf3.py b/src/osmo_gsm_tester/iperf3.py
index e1ed6ff..894b10f 100644
--- a/src/osmo_gsm_tester/iperf3.py
+++ b/src/osmo_gsm_tester/iperf3.py
@@ -105,7 +105,7 @@
             self.process = process.NetNSProcess(self.name(), self.run_dir, netns, popen_args, env={})
         else:
             self.process = process.Process(self.name(), self.run_dir, popen_args, env={})
-        process.run_proc_sync(self.process)
+        self.process.launch_sync()
         return self.get_results()
 
     def get_results(self):