t: port helper/test-oid-array.c to unit-tests/t-oid-array.c

helper/test-oid-array.c along with t0064-oid-array.sh test the
oid-array.h API, which provides storage and processing
efficiency over large lists of object identifiers.

Migrate them to the unit testing framework for better runtime
performance and efficiency. As we don't initialize a repository
in these tests, the hash algo that functions like oid_array_lookup()
use is not initialized, therefore call repo_set_hash_algo() to
initialize it. And init_hash_algo():lib-oid.c can aid in this
process, so make it public.

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

t/unit-tests/lib-oid.c

@@ -3,7 +3,7 @@
 #include "strbuf.h"
 #include "hex.h"
 
-static int init_hash_algo(void)
+int init_hash_algo(void)
 {
 	static int algo = -1;