[testapp] Re-add testapp to default manifest

This commit re-adds testapp to the default manifest and does some
housecleaning on the module:

- Remove the old serial.* and io.*
- Update it to use current syscall APIs
- Update it to use libj6's higher-level thread API

src/libraries/j6/thread.cpp

@@ -45,6 +45,12 @@ thread::start()
     return m_status;
 }
 
+void
+thread::join()
+{
+    j6_thread_join(m_thread);
+}
+
 }
 
 #endif // __j6kernel