The unit tests for the functions have been added.

modified:   coupler/opc-ua-server/mod_io_i2c.h
        new file:   tests/unit_test/Makefile
        new file:   tests/unit_test/test_common.c
        new file:   tests/unit_test/test_modio_i2c.c
        new file:   tests/unit_test/unit_test.md

Test: Program is tested