[webapi][XWALK-1736] Add test cases for download webapi

- Failure analysis: This feature has not been supported

Impacted tests(approved): new 23, update 0, delete 0
Unit test platform: [Android]
Unit test result summary: pass 0, fail 23, block 0
These are fake links. Why not use a self-description name to show you are intended to do it? And `var` is expected.