Added read-write output streams for unit tests

- Simplified CMService_MaxMind:
- Set output stream to CM_OutputStream_Null if none given
- Removed helper methods _writeError(), _writeErrorLine() and _writeOutputLine()
- Added method _getErrorStream() for mocking
- Kept unit tests output clean in CMService_MaxMindTest:
- Used CM_OutputStream_Null as error stream
- Used CM_OutputStream_Stream_ReadWrite_TemporaryFile as output stream