[usecase-webapi] Add subcase in tests xml

- Add subcase attribute in <testcase>
- Remove redundant elements in tests xml

Impacted tests(approved): new 0, update 3, delete 0
Unit test platform: [Android]
Unit test result summary: pass 1, fail 2, block 0
Merge pull request #719 from haoxli/haoxli/webapi-usecase-w3c/add-subcase

[usecase-webapi] Add subcase in tests xml