[embedding][XWALK-1052] Leverage unit tests about XWalkExtension interface

- Add 3 cases about tests for extension api according to the developer's unit tests.

Impacted tests(approved): new 3, update 0, delete 0
Unit test platform: [Android]
Unit test result summary: pass 3, fail 0, block 0
Merge pull request #645 from zhuyongyong/yongzhu/embeddingAPI/leverage-unitTest

[embedding][XWALK-1052] Leverage unit tests about extension interface