t: move tests from reftable/record_test.c to the new unit test

t: move tests from reftable/record_test.c to the new unit test

common_prefix_size(), get_be24() and put_be24() are functions defined
in reftable/basics.{c, h}. Move the tests for these functions from
reftable/record_test.c to the newly ported test.

Mentored-by: Patrick Steinhardt <ps@pks.im>
Mentored-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Chandra Pratap <chandrapratap3519@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>