t-reftable-record: add reftable_record_cmp() tests for log records

t-reftable-record: add reftable_record_cmp() tests for log records

In the current testing setup for log records, only
reftable_log_record_equal() among log record's comparison functions
is tested.

Modify the existing tests to exercise reftable_log_record_cmp_void()
(using the wrapper function reftable_record_cmp()) alongside
reftable_log_record_equal().
Note that to achieve this, we'll need to replace instances of
reftable_log_record_equal() with the wrapper function
reftable_record_equal().

Rename the now modified test to reflect its nature of exercising
all comparison operations, not just equality.

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>