[webapi][XWALK-522] Optimize the test cases for promises

Impacted tests(approved): new 1, update 3, delete 0
Unit test platform: [Android]
Unit test result summary: pass 4, fail 0, block 0
Merge pull request #530 from zhuyongyong/yongzhu/promise-optimize

[webapi][XWALK-522] Optimize the test cases for promises