Rearrange mock object call

To conform with the rest of "serial_hsit_slave_tests.c" and the order
in which the functions are called by the code
Revert "Makefile improvement"

This reverts commit ee922612db18a7cfccb12e80b1b20d945f488bf9.
The reverted commit excluded the Serial HSIT Unit test suite from the
UNIT_TEST variant