test: enhance the google unit tests of the client

1. Move JAMI_DATA_HOME, JAMI_CONFIG_HOME, JAMI_CACHE_HOME
   setup into individual test main entry
2. Use google test assertion macros instead of Qt
3. Add QT_QPA_FONTDIR to avoid QFont warnings
4. Tests will now fail if the env variables cannot be set

Change-Id: I63f6498d26b646d8ebc60799d8797fd5f13b54f9