[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
Think about t.step_func here and t.unreached_func for onRejected

See https://github.com/w3c/testharness.js/blob/master/docs/api.md#asynchronous-tests