[webapi] Improve 3 cases for Selectors API Level2

Impacted tests(approved): new 0, delete 0, update 3
Unit test platform: [Android][IVI]
Unit test result summary: pass 3, fail 0, block 0
Merge pull request #998 from kangxu/kangxu/webapi/tct-selectorslevel2-w3c-tests/updateCases

[webapi] Improve 3 cases for Selectors API Level 2