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 <redacted>
Mentored-by: Christian Couder <redacted>
Signed-off-by: Chandra Pratap <redacted>
Acked-by: Karthik Nayak <redacted>
Signed-off-by: Junio C Hamano <redacted>