[ivi-tct][webapi] Integrate 1 test for drag and drop from w3c

- Add addElement of DataTransfer interface
- Test come from https://github.com/w3c/web-platform-tests/tree/master/html/editing/dnd/overlay (commit: c68a7f796eb7df702becafadb64df8b930093bb2)
- Update xhtml file to html file


Impacted tests(approved): new 1, update 0, delete 0
Unit test platform: [IVI] crosswalk-10.39.233, t-e-c 0.120
Unit test result summary: pass 1, fail 0, block 0