Feature/bacnet bacerror unit tests (#407)

* Add <zephyr/fff.h> from zephyr v3.1.0

Zephyr is deprecating the zmock library in favor of using
the FFF faking library for defining and controlling fake
implementations of depended-upon APIs called by the code-under-test.

Signed-off-by: Gregory Shue <gregory.shue@legrand.com>

* Fix bacerror_decode_service_request return value; add unit tests

bacerror_decode_service_request() return value now includes
the apdu bytes parsed for invoke_id and service.

Also added a unit test for functions in bacerror.c, using `fff`
for faking the depended-upon functions.

Verified by:

1. (Pass) cd $bacnet-stack/test && make test

2. (Pass) west build -p always -b unit_testing \
              bacnet-stack/zephyr/tests/unit/bacnet/bacerror/ && \
              ./build/testbinary

3. (Pass) ./zephyr/scripts/twister -p unit_testing  \
              -T bacnet-stack/zephyr/tests/unit/bacnet/bacerror/

Signed-off-by: Gregory Shue <gregory.shue@legrand.com>

---------

Signed-off-by: Gregory Shue <gregory.shue@legrand.com>
Co-authored-by: Gregory Shue <gregory.shue@legrand.com>

test/CMakeLists.txt

@@ -33,6 +33,7 @@ add_custom_command(TARGET lcov
 #
 
 list(APPEND testdirs
+  unit/bacnet/bacerror
   unit/bacnet/bits
   )