t/: migrate helper/test-example-decorate to the unit testing framework

t/: migrate helper/test-example-decorate to the unit testing framework

helper/test-example-decorate.c along with t9004-example.sh provide
an example of how to use the functions in decorate.h (which provides
a data structure that associates Git objects to void pointers) and
also test their output.

Migrate them to the new unit testing framework for better debugging
and runtime performance.

Mentored-by: Christian Couder <chriscool@tuxfamily.org>
Mentored-by: Kaartic Sivaraam <kaartic.sivaraam@gmail.com>
Signed-off-by: Ghanshyam Thakkar <shyamthakkar001@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>