nas_test_common: call NAS layer from stack thread context

since NAS layers insn't thread safe, all calls must come from
the same thread

srsue/hdr/stack/upper/test/nas_test_common.h

@@ -155,7 +155,7 @@ public:
   }
   bool switch_on()
   {
-    nas->switch_on();
+    task_sched.defer_task([this]() { nas->switch_on(); });
     return true;
   }
   void write_sdu(uint32_t lcid, srsran::unique_byte_buffer_t sdu) { pdcp->write_sdu(lcid, std::move(sdu)); }