t-reftable-block: use reftable_record_equal() instead of check_str()

t-reftable-block: use reftable_record_equal() instead of check_str()

In the current testing setup, operations like read and write for
reftable blocks as defined by reftable/block.{c, h} are verified by
comparing only the keys of input and output reftable records. This is
not ideal because there can exist inequal reftable records with the
same key. Use the dedicated function for record comparison,
reftable_record_equal(), instead of key-based comparison.

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>