[webapi][XWALK-3204] Delete the duplicated case ID

- Delete the duplicated case ID background-002
[webapi] Format tests.xml and tests.full.xml

- Format tests.xml by 'xmllint --format tests.xml'
- Format tests.full.xml by 'xmllint --format tests.full.xml'
Merge pull request #1519 from chenxix/webapi/modify-background

[webapi][XWALK-3204] Delete the duplicated case ID