[cordova]  Add CordovaMobileSpec in tests.xml with comment

- Add CordovaMobileSpec in tests.xml, but comment it, and update README for declear
- Update CordovaMobileSpec test description
you'd better tell the user which package contains the lib. e.g. https://download.01.org/crosswalk/releases/crosswalk/android/canary/8.36.173.0/x86/crosswalk-apks-8.36.173.0-x86.zip