[embedding][XWALK-2684] Add test cases for loading xhtml file

- Failure analysis: EmbeddingAPI can't support loading url of local xhtml file.

Impacted tests(approved): new 3, update 0, delete 0
Unit test platform: [Android]
Unit test result summary: pass 2, fail 1, block 0
Merge pull request #1010 from zhuyongyong/yongzhu/embeddingAPI/loadXhtml

[embedding][XWALK-2684] Add test cases for loading xhtml file