tests: revert XInitThreads() in yamidecode -- need more investigation

- it breaks yamidecode test, X11 call blocks at XGetWindowAttributes()
  after va dlopen() i965 driver. it is strange.
- mp4/h264 files still breaks in yamidecode
- however, mp4/h264 files can work in v4l2decode test
seems, yamidecode is single thread, we needn't call XInitThreads().