Added test for draggable attribute.

[sunling]

Added test for draggable attribute.

[hoppipolla-critic-bot]

Critic review: https://critic.hoppipolla.co.uk/r/409

This is an external review system which you may optionally use for the code review of your pull request.

[sunling]

@hoppipolla-critic-bot.I am sorry,I am confused about your comment.what would you like me to do?

[zqzhang]

@sunling , hoppipolla critic is just a review system combined to this project. Review comments may be there and should be addressed by you.

So far, @Ms2ger has 2 comments at https://critic.hoppipolla.co.uk/showcomments?review=409&filter=open-issues, which needs your action.

[zqzhang]

And please check if your tests are covered by #280, thanks.

[sunling]

Thanks a lot,if it is covered,How should I do?

[zqzhang]

If your tests have been covered, just close this PR :)

[sunling]

Sorry again,Before I close this PR,Actually I am not sure if this test is covered or not.I have no access to see any code in #280

[zqzhang]

Could you please check https://github.com/w3c/web-platform-tests/pull/280/files or http://w3c-test.org/web-platform-tests/submissions/280/html/editing/dnd/the-draggable-attribute/draggable_attribute.html?

[sunling]

Yes,I can.Thanks for your patience!

[sunling]

@zqzhang I have one more question to bother you,I don't know how to close this PR.

[zqzhang]

Under this comment box, there are 2 buttons, one is 'Comment' in green background, the other is 'Close' in light gray background. I think just clicking the 'Close' button is OK.