Wait for server threads to finish

Author: nobu

test/net/http/utils.rb

@@ -41,8 +41,9 @@ def run(socket)
     end
 
     def shutdown
-      @thread.kill if @thread
-      @server.close if @server
+      @thread&.kill
+      @server&.close
+      @thread&.join
     end
 
     def mount(path, proc)