t-reftable-record: add comparison tests for obj records

t-reftable-record: add comparison tests for obj records

In the current testing setup for obj records, the comparison
functions for obj records, reftable_obj_record_cmp_void() and
reftable_obj_record_equal_void() are left untested.

Add tests for the same by using the wrapper functions
reftable_record_cmp() and reftable_record_equal() for
reftable_index_record_cmp_void() and reftable_index_record_equal_void()
respectively.

Mentored-by: Patrick Steinhardt <ps@pks.im>
Mentored-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Chandra Pratap <chandrapratap3519@gmail.com>
Acked-by: Karthik Nayak <karthik.188@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>