Adding threaded blinky integration test

TESTS/integration/threaded_blinky/main.cpp

@@ -0,0 +1,25 @@
+#include "test_env.h"
+#include "mbed.h"
+#include "rtos.h"
+
+DigitalOut led1(LED1);
+
+  
+void led1_thread(void const *args) {
+    int count = 0;
+    while (true) {
+        Thread::wait(1000);
+        greentea_send_kv("tick", count);
+        count++;
+        led1 = !led1;
+    }
+}
+ 
+int main() {
+    GREENTEA_SETUP(20, "wait_us_auto");
+    
+    Thread thread(led1_thread);
+    
+    while (true) {
+    }
+}