tests/unit: add test-fifo unit test

tests/unit: add test-fifo unit test

This tests the Fifo8 implementation basic operations as well as
testing the *_bufptr() in-place buffer functions and the newer
*_buf() functions that also handle wraparound of the internal
FIFO buffer.

Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Reviewed-by: Octavian Purdila <tavip@google.com>
Acked-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Tested-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-ID: <20240828122258.928947-10-mark.cave-ayland@ilande.co.uk>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>