Stop 'mpg123' after test

mpg123 keep running after test until stop_mpg123 called.
Following is `top` log after test(original code). CPU 100%!!

````
  PID USER      PR  NI  VIRT  RES  SHR S  %CPU %MEM    TIME+  COMMAND           
28994 syohei    20   0 99404 2792 2052 R  99.8  0.0   0:05.45 mpg123            
````