ppp-test: check if ppp interface is active and IP address correct before starting the test

Addressing stability of pppos test:
* listing active network interfaces for presence of "ppp0" interface
* fixed IPv6 address regex to prevent ttfw accept partial address only
(less than 8 octets)
* workaround crash after exiting app_main()

tools/test_apps/protocols/pppos/app_test.py

@@ -5,6 +5,7 @@ import socket
 import subprocess
 import ttfw_idf
 import time
+import netifaces
 from threading import Thread, Event
 
 
@@ -58,7 +59,13 @@ def test_examples_protocol_pppos_connect(env, extra_data):
     t = Thread(target=run_server, args=(server_stop, port, server_ip, client_ip))
     t.start()
     try:
-        ip6_addr = dut1.expect(re.compile(r"Got IPv6 address ([0-9a-f\:]+)"), timeout=30)[0]
+        ppp_server_timeout = time.time() + 30
+        while "ppp0" not in netifaces.interfaces():
+            print("PPP server haven't yet setup its netif, list of active netifs:{}".format(netifaces.interfaces()))
+            time.sleep(0.5)
+            if time.time() > ppp_server_timeout:
+                raise ValueError("ENV_TEST_FAILURE: PPP server failed to setup ppp0 interface within timeout")
+        ip6_addr = dut1.expect(re.compile(r"Got IPv6 address (\w{4}\:\w{4}\:\w{4}\:\w{4}\:\w{4}\:\w{4}\:\w{4}\:\w{4})"), timeout=30)[0]
         print("IPv6 address of ESP: {}".format(ip6_addr))
 
         dut1.expect(re.compile(r"Socket listening"))

tools/test_apps/protocols/pppos/main/pppos_client_main.c

@@ -247,6 +247,8 @@ void app_main(void)
         ESP_LOGE(TAG, "IPv6 test failed!");
     }
 
+    /* Wait for the PPP server to stop */
+    vTaskDelay(30000 / portTICK_PERIOD_MS);
     ESP_ERROR_CHECK(esp_modem_stop_ppp(dte));
     /* Destroy the netif adapter withe events, which internally frees also the esp-netif instance */
     esp_modem_netif_clear_default_handlers(modem_netif_adapter);