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

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

In the current testing setup for index records, the comparison
functions for index records, reftable_index_record_cmp() and
reftable_index_record_equal() 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() and reftable_index_record_equal()
respectively.

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>