t/: migrate helper/test-{sha1, sha256} to unit-tests/t-hash

t/: migrate helper/test-{sha1, sha256} to unit-tests/t-hash

t/helper/test-{sha1, sha256} and t/t0015-hash.sh test the hash
implementation of SHA-1 and SHA-256 in Git with basic hash values.
Migrate them to the new unit testing framework for better debugging
and runtime performance.

The 'sha1' and 'sha256' subcommands are still not removed due to
pack_trailer():lib-pack.sh's reliance on them. The 'sha1' subcommand
is also relied upon by t0013-sha1dc (which requires 'test-tool
sha1' dying when it is used on a file created to contain the
known sha1 attack).

Helped-by: Patrick Steinhardt <ps@pks.im>
Mentored-by: Christian Couder <chriscool@tuxfamily.org>
Mentored-by: Kaartic Sivaraam <kaartic.sivaraam@gmail.com>
Co-authored-by: Achu Luma <ach.lumap@gmail.com>
Signed-off-by: Achu Luma <ach.lumap@gmail.com>
Signed-off-by: Ghanshyam Thakkar <shyamthakkar001@gmail.com>
Acked-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>