t-reftable-record: add tests for reftable_log_record_compare_key()

t-reftable-record: add tests for reftable_log_record_compare_key()

reftable_log_record_compare_key() is a function defined by
reftable/record.{c, h} and is used to compare the keys of two
log records when sorting multiple log records using 'qsort'.
In the current testing setup, this function is left unexercised.
Add a testing function for the same.

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>