Make starter_test reliable

We were setting a success bool in another thread, but weren't
synchronizing with that thread.  Join the thread before reading the
success variable to synchronize that variable.

This makes starter_test go from 1-2% failure to 0% failure on my
machine.

Change-Id: I1a54d2fded7800e4ca1576accbfab06bcfb36679
Signed-off-by: Austin Schuh <austin.linux@gmail.com>
diff --git a/aos/starter/starter_test.cc b/aos/starter/starter_test.cc
index 79880f7..94566ae 100644
--- a/aos/starter/starter_test.cc
+++ b/aos/starter/starter_test.cc
@@ -176,9 +176,9 @@
   client_started.Wait();
 
   watcher_loop.Run();
-  ASSERT_TRUE(success);
   test_done_ = true;
   client_thread.join();
+  ASSERT_TRUE(success);
   starterd_thread.join();
 }