test: fix socket issue in iperf example test

examples/wifi/iperf/components/iperf/iperf.c

@@ -149,6 +149,7 @@ static esp_err_t IRAM_ATTR iperf_run_tcp_server(void)
         return ESP_FAIL;
     }
 
+    printf("iperf tcp server create successfully\n");
     buffer = s_iperf_ctrl.buffer;
     want_recv = s_iperf_ctrl.buffer_len;
     while (!s_iperf_ctrl.finish) {

examples/wifi/iperf/iperf_test.py

@@ -358,6 +358,12 @@ class IperfTestUtility(object):
             with open(PC_IPERF_TEMP_LOG_FILE, "w") as f:
                 if proto == "tcp":
                     self.dut.write("iperf -s -i 1 -t {}".format(TEST_TIME))
+                    # wait until DUT TCP server created
+                    try:
+                        self.dut.expect("iperf tcp server create successfully", timeout=1)
+                    except DUT.ExpectTimeout:
+                        # compatible with old iperf example binary
+                        pass
                     process = subprocess.Popen(["iperf", "-c", dut_ip,
                                                 "-t", str(TEST_TIME), "-f", "m"],
                                                stdout=f, stderr=f)