Improve unit test coverage

[jchabloz]

The verification coverage at unit level should be improved. More specifically, error cases (e.g. a NULL pointer is returned) are mostly not covered yet. In order to ease the unit test verification for thoses cases, mock / fake functions should be used.
The current unit test framwork used is CUnit and coverage figures are gathered using lcov.
It is proposed to use Fake function framework for the purpose of using fakes.