pytest: Expect "Calling app_main()" to indicate start of application

This commit updates variousf pytest scripts to expect
"main_task: Calling app_main()" instead of "cpu_start: Starting scheduler" as
indicator of the start of an application.

components/freertos/app_startup.c

@@ -199,6 +199,10 @@ static void main_task(void* args)
     ESP_ERROR_CHECK(esp_task_wdt_init(&twdt_config));
 #endif // CONFIG_ESP_TASK_WDT
 
+    /*
+    Note: Be careful when changing the "Calling app_main()" log below as multiple pytest scripts expect this log as a
+    start-of-application marker.
+    */
     ESP_LOGI(MAIN_TAG, "Calling app_main()");
     extern void app_main(void);
     app_main();