t-reftable-merged: add tests for reftable_merged_table_max_update_index

t-reftable-merged: add tests for reftable_merged_table_max_update_index

reftable_merged_table_max_update_index() as defined by reftable/
merged.{c, h} returns the maximum update index in a merged table.
Since this function is currently unexercised, add tests for it.

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>