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

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

helper/test-hashmap.c along with t0011-hashmap.sh test the hashmap.h
library. Migrate them to the unit testing framework for better
debugging, runtime performance and concise code.

Along with the migration, make 'add' tests from the shell script order
agnostic in unit tests, since they iterate over entries with the same
keys and we do not guarantee the order. This was already done for the
'iterate' tests[1].

The helper/test-hashmap.c is still not removed because it contains a
performance test meant to be run by the user directly (not used in
t/perf). And it makes sense for such a utility to be a helper.

[1]: e1e7a77141 (t: sort output of hashmap iteration, 2019-07-30)

Mentored-by: Christian Couder <chriscool@tuxfamily.org>
Mentored-by: Kaartic Sivaraam <kaartic.sivaraam@gmail.com>
Helped-by: Josh Steadmon <steadmon@google.com>
Helped-by: Junio C Hamano <gitster@pobox.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>