Add unit tests for `FlogCli` opaque type and related functions

[marcransome]

Unit tests should be added for the opaque FlogCli type and its related functions.

The macOS linker (ld64) lacks support for the --wrap option provided by the GNU linker (ld) meaning symbol wrapping is not an option with the cmocka unit test framework for mocking objects. An alternative method is required for mocking in order to test the FlogCli opaque type, potentially using conditional preprocessor directives.