t: migrate helper/test-oidmap.c to unit-tests/t-oidmap.c

t: migrate helper/test-oidmap.c to unit-tests/t-oidmap.c

helper/test-oidmap.c along with t0016-oidmap.sh test the oidmap.h
library which is built on top of hashmap.h.

Migrate them to the unit testing framework for better performance,
concise code and better debugging. Along with the migration also plug
memory leaks and make the test logic independent for all the tests.
The migration removes 'put' tests from t0016, because it is used as
setup to all the other tests, so testing it separately does not yield
any benefit.

Mentored-by: Christian Couder <chriscool@tuxfamily.org>
Mentored-by: Kaartic Sivaraam <kaartic.sivaraam@gmail.com>
Reviewed-by: Josh Steadmon <steadmon@google.com>
Helped-by: Phillip Wood <phillip.wood123@gmail.com>
Helped-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Ghanshyam Thakkar <shyamthakkar001@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>