Fix decode_signed32(); Add unit/bacnet/bacint tests (#392)

Fix decode_signed32() length for -8388608 and NULL apdu handling.

Add comprehensive unit tests for bacint.c functionality.

Verified by:

1. (Pass) From $bacnet-stack/test:
            make clean test
2. (Pass) From $workspace (above $bacnet-stack):
            west build -p always -b unit_testing \
              bacnet-stack/zephyr/tests/unit/bacnet/bacint/ && \
            ./build/testbinary

3. (Pass) From $workspace (above $bacnet-stack):
            ./zephyr/scripts/twister -p unit_testing \
              -T bacnet-stack/zephyr/tests/unit/bacnet/bacint/

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

test/unit/bacnet/bacint/CMakeLists.txt

@@ -0,0 +1,41 @@
+# SPDX-License-Identifier: MIT
+
+cmake_minimum_required(VERSION 3.10 FATAL_ERROR)
+
+get_filename_component(basename ${CMAKE_CURRENT_SOURCE_DIR} NAME)
+project(test_${basename}
+	VERSION 1.0.0
+	LANGUAGES C)
+
+
+string(REGEX REPLACE
+    "/test/unit/bacnet/[a-zA-Z_/-]*$"
+    "/src"
+    SRC_DIR
+    ${CMAKE_CURRENT_SOURCE_DIR})
+string(REGEX REPLACE
+    "/test/unit/bacnet/[a-zA-Z_/-]*$"
+    "/test"
+    TST_DIR
+    ${CMAKE_CURRENT_SOURCE_DIR})
+set(ZTST_DIR "${TST_DIR}/ztest/src")
+
+add_compile_definitions(
+	BIG_ENDIAN=0
+	CONFIG_ZTEST=1
+	)
+
+include_directories(
+	${SRC_DIR}
+	${TST_DIR}/ztest/include
+	)
+
+add_executable(${PROJECT_NAME}
+    # File(s) under test
+    #   NOTE: bits.h contains the complete implementation of the API.
+    # Support files and stubs (pathname alphabetical)
+    # Test and test library files
+	./src/main.c
+	${ZTST_DIR}/ztest_mock.c
+	${ZTST_DIR}/ztest.c
+	)