Refactor the gtest code because it confused me while adding new tests.
This patch focus on refining utility function but it also contains other
minor refinements.
Changes includes:
1. Add comments to utility function
2. Move common utility functions to Common.cpp and remove duplicates
3. Header file removal and reorder
4. Unify MPL commnetc
5. Replace anonymouse namespace with static function
Differential Revision: https://phabricator.services.mozilla.com/D37532
0a6035c0c3