iperf3: add getter to read the actual run time of an iperf client run

this is helpful to run some action for the exact run time of an iperf3
session. note that if no time spec is given when calling
"prepare_test_proc()", a modifier could set the runtime of an DL experiment
to an arbitrary value so we need to have a mechanism to read
the actual time from within a test

Change-Id: I484a0add322ffd269a1e9df72a00cf348950b268

src/osmo_gsm_tester/obj/iperf3.py

@@ -209,6 +209,7 @@ class IPerf3Client(log.Origin):
         self.server = iperf3srv
         self.testenv = testenv
         self._proto = None
+        self._time_sec = None
         self.log_file = None
         self.rem_host = None
         self.remote_log_file = None
@@ -237,6 +238,7 @@ class IPerf3Client(log.Origin):
             else:
                 time_sec = int(time_sec_str)
         assert(time_sec)
+        self._time_sec = time_sec
 
         if proto is None:
             proto = values.get('protocol', IPerf3Client.PROTO_TCP)
@@ -324,6 +326,9 @@ class IPerf3Client(log.Origin):
     def proto(self):
         return self._proto
 
+    def time_sec(self):
+        return self._time_sec
+
     def __str__(self):
         # FIXME: somehow differentiate between several clients connected to same server?
         return "%s:%u" %(self.server.addr(), self.server.port())