X-origin track test

To test whether a cross-origin track can be loaded, modify the code from e.g. w3schools video example to the following:

<video width="320" height="240" controls crossorigin>
  <source src="movie.mp4" type="video/mp4">
  <source src="movie.ogg" type="video/ogg">
  <track kind="captions" src="https://chaals.github.io/testcases/webvtt-file.vtt">
  Your browser does not support the video tag.
</video>